feat: Add active users prometheus metric (#3406)

This  allows deployments using our Prometheus export t determine
the number of active users in the past hour.

The interval is an hour to align with API key last used refresh times.

SSH connections poll to check shutdown time, so this will be accurate
even on long-running connections without dashboard requests.

coderd/coderd.go

@@ -16,6 +16,7 @@ import (
 	"github.com/go-chi/chi/v5/middleware"
 	"github.com/klauspost/compress/zstd"
 	"github.com/pion/webrtc/v3"
+	"github.com/prometheus/client_golang/prometheus"
 	sdktrace "go.opentelemetry.io/otel/sdk/trace"
 	"golang.org/x/xerrors"
 	"google.golang.org/api/idtoken"
@@ -58,6 +59,7 @@ type Options struct {
 	GoogleTokenValidator *idtoken.Validator
 	GithubOAuth2Config   *GithubOAuth2Config
 	OIDCConfig           *OIDCConfig
+	PrometheusRegistry   *prometheus.Registry
 	ICEServers           []webrtc.ICEServer
 	SecureAuthCookie     bool
 	SSHKeygenAlgorithm   gitsshkey.Algorithm
@@ -87,6 +89,9 @@ func New(options *Options) *API {
 			panic(xerrors.Errorf("rego authorize panic: %w", err))
 		}
 	}
+	if options.PrometheusRegistry == nil {
+		options.PrometheusRegistry = prometheus.NewRegistry()
+	}
 
 	siteCacheDir := options.CacheDir
 	if siteCacheDir != "" {
@@ -116,7 +121,7 @@ func New(options *Options) *API {
 				next.ServeHTTP(middleware.NewWrapResponseWriter(w, r.ProtoMajor), r)
 			})
 		},
-		httpmw.Prometheus,
+		httpmw.Prometheus(options.PrometheusRegistry),
 		tracing.HTTPMW(api.TracerProvider, "coderd.http"),
 	)