process metrics on darwin

Adds support for metrics:
- process_start_time_seconds
- process_cpu_seconds_total
- process_virtual_memory_max_bytes
- process_open_fds
- process_max_fds

Fixes #75

Inspired by
https://github.com/prometheus/client_golang/blob/main/prometheus/process_collector_darwin.go

Author: makasim

process_metrics_darwin.go

@@ -0,0 +1,93 @@
+//go:build darwin && !ios
+
+package metrics
+
+import (
+	"io"
+	"log"
+	"os"
+	"syscall"
+	"time"
+
+	"golang.org/x/sys/unix"
+)
+
+func writeProcessMetrics(w io.Writer) {
+	if procs, err := unix.SysctlKinfoProcSlice("kern.proc.pid", os.Getpid()); err == nil {
+		if len(procs) == 1 {
+			startTime := float64(procs[0].Proc.P_starttime.Nano() / 1e9)
+			WriteGaugeUint64(w, "process_start_time_seconds", uint64(startTime))
+		} else {
+			log.Printf("ERROR: metrics: sysctl() returned %d proc structs (expected 1)", len(procs))
+		}
+	} else {
+		log.Printf("ERROR: metrics: %s", err)
+	}
+
+	// The proc structure returned by kern.proc.pid above has an Rusage member,
+	// but it is not filled in, so it needs to be fetched by getrusage(2).  For
+	// that call, the UTime, STime, and Maxrss members are filled out, but not
+	// Ixrss, Idrss, or Isrss for the memory usage.  Memory stats will require
+	// access to the C API to call task_info(TASK_BASIC_INFO).
+	rusage := unix.Rusage{}
+
+	if err := unix.Getrusage(syscall.RUSAGE_SELF, &rusage); err == nil {
+		cpuTime := time.Duration(rusage.Stime.Nano() + rusage.Utime.Nano()).Seconds()
+		WriteGaugeFloat64(w, "process_cpu_seconds_total", cpuTime)
+	} else {
+		log.Printf("ERROR: metrics: %s", err)
+	}
+
+	if addressSpace, err := getSoftLimit(syscall.RLIMIT_AS); err == nil {
+		WriteGaugeFloat64(w, "process_virtual_memory_max_bytes", float64(addressSpace))
+	} else {
+		log.Printf("ERROR: metrics: %s", err)
+	}
+}
+
+func writeFDMetrics(w io.Writer) {
+	if fds, err := getOpenFileCount(); err == nil {
+		WriteGaugeFloat64(w, "process_open_fds", fds)
+	} else {
+		log.Printf("ERROR: metrics: %s", err)
+	}
+
+	if openFiles, err := getSoftLimit(syscall.RLIMIT_NOFILE); err == nil {
+		WriteGaugeFloat64(w, "process_max_fds", float64(openFiles))
+	} else {
+		log.Printf("ERROR: metrics: %s", err)
+	}
+}
+
+func getOpenFileCount() (float64, error) {
+	// Alternately, the undocumented proc_pidinfo(PROC_PIDLISTFDS) can be used to
+	// return a list of open fds, but that requires a way to call C APIs.  The
+	// benefits, however, include fewer system calls and not failing when at the
+	// open file soft limit.
+
+	if dir, err := os.Open("/dev/fd"); err != nil {
+		return 0.0, err
+	} else {
+		defer dir.Close()
+
+		// Avoid ReadDir(), as it calls stat(2) on each descriptor.  Not only is
+		// that info not used, but KQUEUE descriptors fail stat(2), which causes
+		// the whole method to fail.
+		if names, err := dir.Readdirnames(0); err != nil {
+			return 0.0, err
+		} else {
+			// Subtract 1 to ignore the open /dev/fd descriptor above.
+			return float64(len(names) - 1), nil
+		}
+	}
+}
+
+func getSoftLimit(which int) (uint64, error) {
+	rlimit := syscall.Rlimit{}
+
+	if err := syscall.Getrlimit(which, &rlimit); err != nil {
+		return 0, err
+	}
+
+	return rlimit.Cur, nil
+}

process_metrics_other.go

@@ -1,4 +1,4 @@
-//go:build !linux && !windows && !solaris
+//go:build !linux && !windows && !solaris && !darwin
 
 package metrics