prober: migrate to Prometheus metric library

This provides an example of using native Prometheus metrics with tsweb.

Prober library seems to be the only user of PrometheusVar, so I am
removing support for it in tsweb.

Updates https://github.com/tailscale/corp/issues/10205

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

tsweb/tsweb.go

@@ -448,15 +448,6 @@ func Error(code int, msg string, err error) HTTPError {
 	return HTTPError{Code: code, Msg: msg, Err: err}
 }
 
-// PrometheusVar is a value that knows how to format itself into
-// Prometheus metric syntax.
-type PrometheusVar interface {
-	// WritePrometheus writes the value of the var to w, in Prometheus
-	// metric syntax. All variables names written out must start with
-	// prefix (or write out a single variable named exactly prefix)
-	WritePrometheus(w io.Writer, prefix string)
-}
-
 // WritePrometheusExpvar writes kv to w in Prometheus metrics format.
 //
 // See VarzHandler for conventions. This is exported primarily for
@@ -510,9 +501,6 @@ func writePromExpVar(w io.Writer, prefix string, kv expvar.KeyValue) {
 	name, typ, label := prometheusMetric(prefix, key)
 
 	switch v := kv.Value.(type) {
-	case PrometheusVar:
-		v.WritePrometheus(w, name)
-		return
 	case *expvar.Int:
 		if typ == "" {
 			typ = "counter"

tsweb/tsweb_test.go

@@ -9,7 +9,6 @@ import (
 	"errors"
 	"expvar"
 	"fmt"
-	"io"
 	"net"
 	"net/http"
 	"net/http/httptest"
@@ -570,12 +569,6 @@ foo_totalY 4
 			expvar.Func(func() any { return 123 }),
 			"num_goroutines 123\n",
 		},
-		{
-			"var_that_exports_itself",
-			"custom_var",
-			promWriter{},
-			"custom_var_value 42\n",
-		},
 		{
 			"string_version_var",
 			"foo_version",
@@ -694,16 +687,6 @@ func (a expvarAdapter2) PrometheusMetricsReflectRoot() any {
 	return a.st
 }
 
-type promWriter struct{}
-
-func (promWriter) WritePrometheus(w io.Writer, prefix string) {
-	fmt.Fprintf(w, "%s_value 42\n", prefix)
-}
-
-func (promWriter) String() string {
-	return ""
-}
-
 func TestAcceptsEncoding(t *testing.T) {
 	tests := []struct {
 		in, enc string