Metrics endpoint for internal pgwatch metrics

[evenhummus]

Is your feature request related to a problem? Please describe.
It would be handy to have a prometheus metrics endpoint from pgwatch itself. Useful metrics would be:

1. How many databases are monitored by pgwatch
2. How many sinks are currently connected and reachable (so we would see when the connection to the timescaldedb/prometheus etc. fails)
3. etc. ...

Describe the solution you'd like
Metrics endpoint (/metrics or /prometheus) which provides internal pgwatch metrics

[pashagolub]

Hello, we have some, e.g. http://localhost:9187/pgwatch will produce

# HELP go_gc_duration_seconds A summary of the wall-time pause (stop-the-world) duration in garbage collection cycles.
# TYPE go_gc_duration_seconds summary
go_gc_duration_seconds{quantile="0"} 4.698e-05
go_gc_duration_seconds{quantile="0.25"} 0.000208854
go_gc_duration_seconds{quantile="0.5"} 0.000234202
go_gc_duration_seconds{quantile="0.75"} 0.000277556
go_gc_duration_seconds{quantile="1"} 0.000277556
go_gc_duration_seconds_sum 0.000767592
go_gc_duration_seconds_count 4
# HELP go_gc_gogc_percent Heap size target percentage configured by the user, otherwise 100. This value is set by the GOGC environment variable, and the runtime/debug.SetGCPercent function. Sourced from /gc/gogc:percent.
# TYPE go_gc_gogc_percent gauge
go_gc_gogc_percent 100
# HELP go_gc_gomemlimit_bytes Go runtime memory limit configured by the user, otherwise math.MaxInt64. This value is set by the GOMEMLIMIT environment variable, and the runtime/debug.SetMemoryLimit function. Sourced from /gc/gomemlimit:bytes.
# TYPE go_gc_gomemlimit_bytes gauge
go_gc_gomemlimit_bytes 9.223372036854776e+18
# HELP go_goroutines Number of goroutines that currently exist.
# TYPE go_goroutines gauge
go_goroutines 21
# HELP go_info Information about the Go environment.
# TYPE go_info gauge
go_info{version="go1.25.4 X:greenteagc"} 1
# HELP go_memstats_alloc_bytes Number of bytes allocated in heap and currently in use. Equals to /memory/classes/heap/objects:bytes.
# TYPE go_memstats_alloc_bytes gauge
go_memstats_alloc_bytes 1.992096e+06
# HELP go_memstats_alloc_bytes_total Total number of bytes allocated in heap until now, even if released already. Equals to /gc/heap/allocs:bytes.
# TYPE go_memstats_alloc_bytes_total counter
go_memstats_alloc_bytes_total 6.225976e+06
# HELP go_memstats_buck_hash_sys_bytes Number of bytes used by the profiling bucket hash table. Equals to /memory/classes/profiling/buckets:bytes.
# TYPE go_memstats_buck_hash_sys_bytes gauge
go_memstats_buck_hash_sys_bytes 1.445509e+06
# HELP go_memstats_frees_total Total number of heap objects frees. Equals to /gc/heap/frees:objects + /gc/heap/tiny/allocs:objects.
# TYPE go_memstats_frees_total counter
go_memstats_frees_total 20258
# HELP go_memstats_gc_sys_bytes Number of bytes used for garbage collection system metadata. Equals to /memory/classes/metadata/other:bytes.
# TYPE go_memstats_gc_sys_bytes gauge
go_memstats_gc_sys_bytes 3.470096e+06
# HELP go_memstats_heap_alloc_bytes Number of heap bytes allocated and currently in use, same as go_memstats_alloc_bytes. Equals to /memory/classes/heap/objects:bytes.
# TYPE go_memstats_heap_alloc_bytes gauge
go_memstats_heap_alloc_bytes 1.992096e+06
# HELP go_memstats_heap_idle_bytes Number of heap bytes waiting to be used. Equals to /memory/classes/heap/released:bytes + /memory/classes/heap/free:bytes.
# TYPE go_memstats_heap_idle_bytes gauge
go_memstats_heap_idle_bytes 2.973696e+06
# HELP go_memstats_heap_inuse_bytes Number of heap bytes that are in use. Equals to /memory/classes/heap/objects:bytes + /memory/classes/heap/unused:bytes
# TYPE go_memstats_heap_inuse_bytes gauge
go_memstats_heap_inuse_bytes 4.46464e+06
# HELP go_memstats_heap_objects Number of currently allocated objects. Equals to /gc/heap/objects:objects.
# TYPE go_memstats_heap_objects gauge
go_memstats_heap_objects 6117
# HELP go_memstats_heap_released_bytes Number of heap bytes released to OS. Equals to /memory/classes/heap/released:bytes.
# TYPE go_memstats_heap_released_bytes gauge
go_memstats_heap_released_bytes 2.203648e+06
# HELP go_memstats_heap_sys_bytes Number of heap bytes obtained from system. Equals to /memory/classes/heap/objects:bytes + /memory/classes/heap/unused:bytes + /memory/classes/heap/released:bytes + /memory/classes/heap/free:bytes.
# TYPE go_memstats_heap_sys_bytes gauge
go_memstats_heap_sys_bytes 7.438336e+06
# HELP go_memstats_last_gc_time_seconds Number of seconds since 1970 of last garbage collection.
# TYPE go_memstats_last_gc_time_seconds gauge
go_memstats_last_gc_time_seconds 1.767363555576264e+09
# HELP go_memstats_mallocs_total Total number of heap objects allocated, both live and gc-ed. Semantically a counter version for go_memstats_heap_objects gauge. Equals to /gc/heap/allocs:objects + /gc/heap/tiny/allocs:objects.
# TYPE go_memstats_mallocs_total counter
go_memstats_mallocs_total 26375
# HELP go_memstats_mcache_inuse_bytes Number of bytes in use by mcache structures. Equals to /memory/classes/metadata/mcache/inuse:bytes.
# TYPE go_memstats_mcache_inuse_bytes gauge
go_memstats_mcache_inuse_bytes 19328
# HELP go_memstats_mcache_sys_bytes Number of bytes used for mcache structures obtained from system. Equals to /memory/classes/metadata/mcache/inuse:bytes + /memory/classes/metadata/mcache/free:bytes.
# TYPE go_memstats_mcache_sys_bytes gauge
go_memstats_mcache_sys_bytes 31408
# HELP go_memstats_mspan_inuse_bytes Number of bytes in use by mspan structures. Equals to /memory/classes/metadata/mspan/inuse:bytes.
# TYPE go_memstats_mspan_inuse_bytes gauge
go_memstats_mspan_inuse_bytes 116160
# HELP go_memstats_mspan_sys_bytes Number of bytes used for mspan structures obtained from system. Equals to /memory/classes/metadata/mspan/inuse:bytes + /memory/classes/metadata/mspan/free:bytes.
# TYPE go_memstats_mspan_sys_bytes gauge
go_memstats_mspan_sys_bytes 130560
# HELP go_memstats_next_gc_bytes Number of heap bytes when next garbage collection will take place. Equals to /gc/heap/goal:bytes.
# TYPE go_memstats_next_gc_bytes gauge
go_memstats_next_gc_bytes 4.194304e+06
# HELP go_memstats_other_sys_bytes Number of bytes used for other system allocations. Equals to /memory/classes/other:bytes.
# TYPE go_memstats_other_sys_bytes gauge
go_memstats_other_sys_bytes 1.302211e+06
# HELP go_memstats_stack_inuse_bytes Number of bytes obtained from system for stack allocator in non-CGO environments. Equals to /memory/classes/heap/stacks:bytes.
# TYPE go_memstats_stack_inuse_bytes gauge
go_memstats_stack_inuse_bytes 950272
# HELP go_memstats_stack_sys_bytes Number of bytes obtained from system for stack allocator. Equals to /memory/classes/heap/stacks:bytes + /memory/classes/os-stacks:bytes.
# TYPE go_memstats_stack_sys_bytes gauge
go_memstats_stack_sys_bytes 950272
# HELP go_memstats_sys_bytes Number of bytes obtained from system. Equals to /memory/classes/total:byte.
# TYPE go_memstats_sys_bytes gauge
go_memstats_sys_bytes 1.4768392e+07
# HELP go_sched_gomaxprocs_threads The current runtime.GOMAXPROCS setting, or the number of operating system threads that can execute user-level Go code simultaneously. Sourced from /sched/gomaxprocs:threads.
# TYPE go_sched_gomaxprocs_threads gauge
go_sched_gomaxprocs_threads 16
# HELP go_threads Number of OS threads created.
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go_threads 12
# HELP pgwatch_exporter_last_scrape_errors Last scrape error count for all monitored hosts / metrics
# TYPE pgwatch_exporter_last_scrape_errors gauge
pgwatch_exporter_last_scrape_errors 0
# HELP pgwatch_exporter_total_scrape_failures Number of errors while executing metric queries
# TYPE pgwatch_exporter_total_scrape_failures counter
pgwatch_exporter_total_scrape_failures 0
# HELP pgwatch_exporter_total_scrapes Total scrape attempts.
# TYPE pgwatch_exporter_total_scrapes counter
pgwatch_exporter_total_scrapes 1
# HELP process_cpu_seconds_total Total user and system CPU time spent in seconds.
# TYPE process_cpu_seconds_total counter
process_cpu_seconds_total 0.12
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process_max_fds 1.048576e+06
# HELP process_network_receive_bytes_total Number of bytes received by the process over the network.
# TYPE process_network_receive_bytes_total counter
process_network_receive_bytes_total 209736
# HELP process_network_transmit_bytes_total Number of bytes sent by the process over the network.
# TYPE process_network_transmit_bytes_total counter
process_network_transmit_bytes_total 236188
# HELP process_open_fds Number of open file descriptors.
# TYPE process_open_fds gauge
process_open_fds 12
# HELP process_resident_memory_bytes Resident memory size in bytes.
# TYPE process_resident_memory_bytes gauge
process_resident_memory_bytes 2.5018368e+07
# HELP process_start_time_seconds Start time of the process since unix epoch in seconds.
# TYPE process_start_time_seconds gauge
process_start_time_seconds 1.76736355493e+09
# HELP process_virtual_memory_bytes Virtual memory size in bytes.
# TYPE process_virtual_memory_bytes gauge
process_virtual_memory_bytes 1.287974912e+09
# HELP process_virtual_memory_max_bytes Maximum amount of virtual memory available in bytes.
# TYPE process_virtual_memory_max_bytes gauge
process_virtual_memory_max_bytes 1.8446744073709552e+19

It's more system connected though