Add metrics package with expvar counters

Create a singleton metrics struct registered with expvar and context
helpers: Set, AddRequests, AddGoroutines, AddErrors, AddPanics.
Goroutine metric is refreshed every 100 requests.

Author: standard-librarian

business/web/v1/mertrics/metrics.go

@@ -0,0 +1,92 @@
+package metrics
+
+import (
+	"context"
+	"expvar"
+	"runtime"
+)
+
+// This holds the single instance of the metrics value needed for
+// collecting metrics. The expvar package is already based on a singleton
+// for the different metrics that are registered with the package so there
+// isn't much choice here.
+var m *metrics
+
+// =============================================================================
+
+// metrics represents the set of metrics we gather. These fields are
+// safe to be accessed concurrently thanks to expvar. No extra abstraction is required.
+type metrics struct {
+	goroutines *expvar.Int
+	requests   *expvar.Int
+	errors     *expvar.Int
+	panics     *expvar.Int
+}
+
+// init constructs the metrics value that will be used to capture metrics.
+// The metrics value is stored in a package level variable since everything
+// inside of expvar is registered as a singleton. The use of once will make
+// sure this initialization only happens once.
+func init() {
+	m = &metrics{
+		goroutines: expvar.NewInt("goroutines"),
+		requests:   expvar.NewInt("requests"),
+		errors:     expvar.NewInt("errors"),
+		panics:     expvar.NewInt("panics"),
+	}
+}
+
+// =============================================================================
+
+type ctxKey int
+
+const key ctxKey = 1
+
+// Set sets the metrics data into the context.
+func Set(ctx context.Context) context.Context {
+	return context.WithValue(ctx, key, m)
+}
+
+// AddGoroutines refreshes the goroutine metric every 100 requests.
+func AddGoroutines(ctx context.Context) int64 {
+	if v, ok := ctx.Value(key).(*metrics); ok {
+		if v.requests.Value()%100 == 0 {
+			g := int64(runtime.NumGoroutine())
+			v.goroutines.Set(g)
+			return g
+		}
+	}
+
+	return 0
+}
+
+// AddRequests increments the request metric by 1.
+func AddRequests(ctx context.Context) int64 {
+	v, ok := ctx.Value(key).(*metrics)
+	if ok {
+		v.requests.Add(1)
+		return v.goroutines.Value()
+	}
+
+	return 0
+}
+
+// AddErrors increments the errors metric by 1.
+func AddErrors(ctx context.Context) int64 {
+	if v, ok := ctx.Value(key).(*metrics); ok {
+		v.errors.Add(1)
+		return v.errors.Value()
+	}
+
+	return 0
+}
+
+// AddPanics increments the panics metric by 1.
+func AddPanics(ctx context.Context) int64 {
+	if v, ok := ctx.Value(key).(*metrics); ok {
+		v.panics.Add(1)
+		return v.panics.Value()
+	}
+
+	return 0
+}