Add toolkit package

Implement a top level package to make it easy to bootstrap an exporter.
* Move the flag package to the top level.
* Add support for `--web.telemetry-path` flag. Defaults to`/metrics`.
* Add a self-check function to the web FlagConfig.

Signed-off-by: SuperQ <superq@gmail.com>

web/kingpinflag/flag.go → flag.go

@@ -10,7 +10,7 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
-package kingpinflag
+package toolkit
 
 import (
 	"runtime"
@@ -31,6 +31,10 @@ func AddFlags(a *kingpin.Application, defaultAddress string) *web.FlagConfig {
 		).Bool()
 	}
 	flags := web.FlagConfig{
+		MetricsPath: a.Flag(
+			"web.telemetry-path",
+			"Path under which to expose metrics.",
+		).Default("/metrics").String(),
 		WebListenAddresses: a.Flag(
 			"web.listen-address",
 			"Addresses on which to expose metrics and web interface. Repeatable for multiple addresses.",

toolkit.go

@@ -0,0 +1,66 @@
+// Copyright 2023 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package toolkit
+
+import (
+	"errors"
+	"net/http"
+	"os"
+
+	"github.com/go-kit/log"
+	"github.com/go-kit/log/level"
+	"github.com/prometheus/exporter-toolkit/web"
+)
+
+var ErrNoFlagConfig = errors.New("Missing FlagConfig")
+
+type Config struct {
+	MetricsHandler     http.Handler
+	MetricsHandlerFunc *func(http.ResponseWriter, *http.Request)
+	FlagConfig         *web.FlagConfig
+	LandingConfig      web.LandingConfig
+	Logger             log.Logger
+}
+
+func Run(c Config) error {
+	if c.FlagConfig == nil {
+		return ErrNoFlagConfig
+	}
+	err := c.FlagConfig.CheckFlags()
+	if err != nil {
+		return err
+	}
+	if *c.FlagConfig.MetricsPath != "" && c.MetricsHandler != nil {
+		http.Handle(*c.FlagConfig.MetricsPath, c.MetricsHandler)
+	}
+	if *c.FlagConfig.MetricsPath != "" && c.MetricsHandlerFunc != nil {
+		http.HandleFunc(*c.FlagConfig.MetricsPath, *c.MetricsHandlerFunc)
+	}
+	if *c.FlagConfig.MetricsPath != "/" && *c.FlagConfig.MetricsPath != "" {
+		c.LandingConfig.Links = append(c.LandingConfig.Links,
+			web.LandingLinks{
+				Address: *c.FlagConfig.MetricsPath,
+				Text:    "Metrics",
+			},
+		)
+		landingPage, err := web.NewLandingPage(c.LandingConfig)
+		if err != nil {
+			level.Error(c.Logger).Log("err", err)
+			os.Exit(1)
+		}
+		http.Handle("/", landingPage)
+	}
+
+	srv := &http.Server{}
+	return web.ListenAndServe(srv, c.FlagConfig, c.Logger)
+}

web/tls_config.go

@@ -34,6 +34,7 @@ import (
 var (
 	errNoTLSConfig = errors.New("TLS config is not present")
 	ErrNoListeners = errors.New("no web listen address or systemd socket flag specified")
+	ErrMissingFlag = errors.New("Flag config is empty")
 )
 
 type Config struct {
@@ -55,11 +56,26 @@ type TLSConfig struct {
 }
 
 type FlagConfig struct {
+	MetricsPath        *string
 	WebListenAddresses *[]string
 	WebSystemdSocket   *bool
 	WebConfigFile      *string
 }
 
+// CheckFlags validates that the FlagConfig has all required values set and has at least one listener.
+func (c *FlagConfig) CheckFlags() error {
+	if c.MetricsPath == nil {
+		return ErrMissingFlag
+	}
+	if c.WebSystemdSocket == nil && (c.WebListenAddresses == nil || len(*c.WebListenAddresses) == 0) {
+		return ErrNoListeners
+	}
+	if c.WebConfigFile == nil {
+		return ErrMissingFlag
+	}
+	return nil
+}
+
 // SetDirectory joins any relative file paths with dir.
 func (t *TLSConfig) SetDirectory(dir string) {
 	t.TLSCertPath = config_util.JoinDir(dir, t.TLSCertPath)
@@ -204,8 +220,9 @@ func ServeMultiple(listeners []net.Listener, server *http.Server, flags *FlagCon
 // WebSystemdSocket in the FlagConfig is true. The FlagConfig is also passed on
 // to ServeMultiple.
 func ListenAndServe(server *http.Server, flags *FlagConfig, logger log.Logger) error {
-	if flags.WebSystemdSocket == nil && (flags.WebListenAddresses == nil || len(*flags.WebListenAddresses) == 0) {
-		return ErrNoListeners
+	err := flags.CheckFlags()
+	if err != nil {
+		return err
 	}
 
 	if flags.WebSystemdSocket != nil && *flags.WebSystemdSocket {