serverconfig: conditionally install expvar handler.

Add local copy of the standard library's expvar handler.

Change-Id: Ida41553345d7d29665f33268356bf16a80386e38

doc/environment-vars.txt

@@ -62,6 +62,9 @@ CAMLI_DEV_KEYBLOBS (string):
 CAMLI_HTTP_DEBUG (bool):
   Enable per-request logging in pkg/webserver.
 
+CAMLI_HTTP_EXPVAR (bool):
+  Enable json export of expvars at /debug/vars
+
 CAMLI_HTTP_PPROF (bool):
   Enable standard library's pprof handler at /debug/pprof/
 

pkg/serverconfig/serverconfig.go

@@ -22,6 +22,7 @@ package serverconfig
 import (
 	"encoding/json"
 	"errors"
+	"expvar"
 	"fmt"
 	"io"
 	"log"
@@ -409,6 +410,9 @@ func (config *Config) InstallHandlers(hi HandlerInstaller, baseURL string, conte
 	}
 	hl.setupAll()
 
+	if v, _ := strconv.ParseBool(os.Getenv("CAMLI_HTTP_EXPVAR")); v {
+		hi.Handle("/debug/vars", expvarHandler{})
+	}
 	if v, _ := strconv.ParseBool(os.Getenv("CAMLI_HTTP_PPROF")); v {
 		hi.Handle("/debug/pprof/", profileHandler{})
 	}
@@ -426,6 +430,23 @@ func (s multiCloser) Close() (err error) {
 	return
 }
 
+// expvarHandler publishes expvar stats.
+type expvarHandler struct{}
+
+func (expvarHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	fmt.Fprintf(w, "{\n")
+	first := true
+	expvar.Do(func(kv expvar.KeyValue) {
+		if !first {
+			fmt.Fprintf(w, ",\n")
+		}
+		first = false
+		fmt.Fprintf(w, "%q: %s", kv.Key, kv.Value)
+	})
+	fmt.Fprintf(w, "\n}\n")
+}
+
 // profileHandler publishes server profile information.
 type profileHandler struct{}