tsweb: make VarzHandler support untyped expvar.Maps for compatibility

Updates #2635

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tsweb/tsweb.go

@@ -438,9 +438,9 @@ func writePromExpVar(w io.Writer, prefix string, kv expvar.KeyValue) {
 		default:
 			fmt.Fprintf(w, "# skipping expvar %q (Go type %T%s) with undeclared Prometheus type\n", name, kv.Value, funcRet)
 			return
-		case *metrics.LabelMap:
+		case *metrics.LabelMap, *expvar.Map:
-			// Permit typeless LabelMap for compatibility
+			// Permit typeless LabelMap and expvar.Map for
-			// with old expvar-registered
+			// compatibility with old expvar-registered
 			// metrics.LabelMap.
 		}
 	}
@@ -471,7 +471,9 @@ func writePromExpVar(w io.Writer, prefix string, kv expvar.KeyValue) {
 				fmt.Fprintf(w, "%s{%s=%q} %v\n", name, label, kv.Key, kv.Value)
 			})
 		} else {
-			fmt.Fprintf(w, "# skipping expvar.Map %q with incomplete metadata: label %q, Prometheus type %q\n", name, label, typ)
+			v.Do(func(kv expvar.KeyValue) {
+				fmt.Fprintf(w, "%s_%s %v\n", name, kv.Key, kv.Value)
+			})
 		}
 	}
 }

tsweb/tsweb_test.go

@@ -363,6 +363,18 @@ func TestVarzHandler(t *testing.T) {
 			},
 			"# TYPE s_bar counter\ns_bar 2\n# TYPE s_foo counter\ns_foo 1\n",
 		},
+		{
+			"expvar_map_untyped",
+			"api_status_code",
+			func() *expvar.Map {
+				m := new(expvar.Map)
+				m.Init()
+				m.Add("2xx", 100)
+				m.Add("5xx", 2)
+				return m
+			}(),
+			"api_status_code_2xx 100\napi_status_code_5xx 2\n",
+		},
 		{
 			"func_float64",
 			"counter_x",
@@ -420,16 +432,6 @@ func TestVarzHandler(t *testing.T) {
 			}(),
 			"# TYPE m counter\nm{keyname=\"bar\"} 2\nm{keyname=\"foo\"} 1\n",
 		},
-		{
-			"expvar_label_map_malformed",
-			"counter_labelmap_lackslabel",
-			func() *expvar.Map {
-				m := new(expvar.Map)
-				m.Init()
-				return m
-			}(),
-			"# skipping expvar.Map \"lackslabel\" with incomplete metadata: label \"\", Prometheus type \"counter\"\n",
-		},
 		{
 			"struct_reflect",
 			"foo",