Introduce metrics.md documentation

Author: pwilczynskiclearcode

docs/metrics.md

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+graph-node provides the following metrics via Prometheus endpoint on 8040 port by default:
+- `deployment_block_processing_duration`
+Measures **duration of block processing** for a subgraph deployment
+- `deployment_block_trigger_count`
+Measures the **number of triggers in each** block for a subgraph deployment
+- `deployment_count` 
+Counts the number of deployments currently being indexed by the graph-node.
+- `deployment_eth_rpc_errors`
+Counts **eth** **rpc request errors** for a subgraph deployment
+- `deployment_eth_rpc_request_duration`
+Measures **eth** **rpc request duration** for a subgraph deployment
+- `deployment_failed`
+Boolean gauge to indicate **whether the deployment has failed** (1 == failed)
+- `deployment_handler_execution_time`
+Measures the **execution time for handlers**
+- `deployment_head`
+Track the **head block number** for a deployment. Example:
+
+```protobuf
+deployment_head{deployment="QmaeWFYbPwmXEk7UuACmkqgPq2Pba5t2RYdJtEyvAUmrxg",network="mumbai",shard="primary"} 19509077
+```
+
+- `deployment_host_fn_execution_time`
+Measures the **execution time for host functions**
+- `deployment_reverted_blocks`
+Track the **last reverted block** for a subgraph deployment
+- `deployment_sync_secs`
+total **time spent syncing**
+- `deployment_transact_block_operations_duration`
+Measures **duration of commiting all the entity operations** in a block and **updating the subgraph pointer**
+- `deployment_trigger_processing_duration`
+Measures **duration of trigger processing** for a subgraph deployment
+- `eth_rpc_errors`
+Counts **eth rpc request errors**
+- `eth_rpc_request_duration`
+Measures **eth rpc request duration**
+- `ethereum_chain_head_number`
+Block **number of the most recent block synced from Ethereum**. Example:
+
+```protobuf
+ethereum_chain_head_number{network="mumbai"} 20045294
+```
+
+- `metrics_register_errors`
+Counts **Prometheus metrics register errors**
+- `metrics_unregister_errors`
+Counts **Prometheus metrics unregister errors**
+- `query_cache_status_count`
+Count **toplevel GraphQL fields executed** and their cache status
+- `query_effort_ms`
+Moving **average of time spent running queries**
+- `query_execution_time`
+**Execution time for successful GraphQL queries**
+- `query_result_max`
+the **maximum size of a query result** (in CacheWeight)
+- `query_result_size` 
+the **size of the result of successful GraphQL queries** (in CacheWeight)
+- `query_semaphore_wait_ms`
+Moving **average of time spent on waiting for postgres query semaphore**
+- `query_kill_rate`
+The rate at which the load manager kills queries
+- `registered_metrics`
+Tracks the **number of registered metrics** on the node
+- `store_connection_checkout_count`
+The **number of Postgres connections** currently **checked out**
+- `store_connection_error_count`
+The **number of Postgres connections errors**
+- `store_connection_wait_time_ms`
+**Average connection wait time**