msp/runtime: add -help handling for config options (#61050)

```
$ go build -o .bin/telemetry-gateway github.com/sourcegraph/sourcegraph/cmd/telemetry-gateway
$ .bin/telemetry-gateway -help                                                                                   
SERVICE: telemetry-gateway
VERSION: 0.0.0+dev
CONFIGURATION OPTIONS:
- 'TELEMETRY_GATEWAY_EVENTS_PUBSUB_ENABLED': If false, logs Pub/Sub messages instead of actually sending them (default: "true")
- 'TELEMETRY_GATEWAY_EVENTS_STREAM_PUBLISH_CONCURRENCY': Per-stream concurrent publishing limit. (default: "250")
- 'MSP': indicates if we are running in a MSP environment (default: "false")
- 'ENVIRONMENT_ID': MSP Service Environment ID (default: "unknown")
- 'PORT': service port (required)
- 'OTEL_SDK_DISABLED': disable OpenTelemetry SDK (default: "false")
```

lib/managedservicesplatform/runtime/env.go

@@ -9,9 +9,19 @@ import (
 	"github.com/sourcegraph/sourcegraph/lib/errors"
 )
 
+type requestedEnvVar struct {
+	name         string
+	defaultValue string
+	description  string
+}
+
 type Env struct {
 	errs []error
 	env  map[string]string
+
+	// requestedEnvVars are only available after ConfigLoader is used on this
+	// Env instance.
+	requestedEnvVars []requestedEnvVar
 }
 
 func newEnv() (*Env, error) {
@@ -30,8 +40,13 @@ func newEnv() (*Env, error) {
 	}, nil
 }
 
-// TODO: Try to use third param description to generate docs.
-func (e *Env) get(name, defaultValue, _ string) string {
+func (e *Env) get(name, defaultValue, description string) string {
+	e.requestedEnvVars = append(e.requestedEnvVars, requestedEnvVar{
+		name:         name,
+		defaultValue: defaultValue,
+		description:  description,
+	})
+
 	v, ok := e.env[name]
 	if !ok {
 		return defaultValue

lib/managedservicesplatform/runtime/service.go

@@ -2,6 +2,9 @@ package runtime
 
 import (
 	"context"
+	"flag"
+	"fmt"
+	"os"
 
 	"cloud.google.com/go/profiler"
 	"github.com/getsentry/sentry-go"
@@ -29,6 +32,8 @@ type Service[ConfigT any] interface {
 	) (background.Routine, error)
 }
 
+var showHelp = flag.Bool("help", false, "Show service help text")
+
 // Start handles the entire lifecycle of the program running Service, and should
 // be the only thing called in a MSP program's main package, for example:
 //
@@ -40,6 +45,7 @@ func Start[
 	ConfigT any,
 	LoaderT ConfigLoader[ConfigT],
 ](service Service[ConfigT]) {
+	flag.Parse()
 	passSanityCheck(service)
 
 	// Resource representing the service
@@ -70,6 +76,23 @@ func Start[
 	config.Load(env)
 	contract := newContract(log.Scoped("msp.contract"), env, service)
 
+	// Fast-exit with configuration facts if requested
+	if *showHelp {
+		fmt.Printf("SERVICE: %s\nVERSION: %s\n",
+			service.Name(), service.Version())
+		fmt.Printf("CONFIGURATION OPTIONS:\n")
+		for _, v := range env.requestedEnvVars {
+			fmt.Printf("- '%s': %s", v.name, v.description)
+			if v.defaultValue != "" {
+				fmt.Printf(" (default: %q)", v.defaultValue)
+			} else {
+				fmt.Printf(" (required)")
+			}
+			fmt.Println()
+		}
+		os.Exit(0)
+	}
+
 	// Enable Sentry error log reporting
 	var sentryEnabled bool
 	if contract.internal.sentryDSN != nil {