Flattening expvars, full example.

flat_expvars_endpoint.go

package main

import (
	"expvar"
	"fmt"
	"io"
	"net/http"
)

func FlattenVar(kv expvar.KeyValue, w io.Writer, prefix string) {
	if m, ok := kv.Value.(*expvar.Map); ok { // If Value is an expvar.Map, append its key to prefix and recurse
		m.Do(func(kv_ expvar.KeyValue) { FlattenVar(kv_, w, fmt.Sprintf("%s%s.", prefix, kv.Key)) })
	} else {
		fmt.Fprintf(w, "\"%s%s\": %s\n", prefix, kv.Key, kv.Value)
	}
}

func main() {
	scalar := expvar.NewInt("scalar")
	scalar.Add(42)

	emap := expvar.NewMap("foo")
	emap.Add("bar", 1)
	emap.Add("bar", 2)

	emap2 := (&expvar.Map{}).Init()
	emap.Set("baz", emap2)
	emap2.Add("frob", 1)

	http.HandleFunc("/debug/cosmos", func(w http.ResponseWriter, r *http.Request) {
		w.Header().Set("content-type", "text/plain")
		expvar.Do(func(kv expvar.KeyValue) {
			if kv.Key == "memstats" || kv.Key == "cmdline" {
				return
			}
			FlattenVar(kv, w, "")
		})
	})

	http.ListenAndServe(":8080", nil)
}