feat: add metrics to PGPubsub (#11971)

Adds prometheus metrics to PGPubsub for monitoring its health and performance in production.

Related to #11950 --- additional diagnostics to help figure out what's happening

cli/server.go

@@ -673,10 +673,14 @@ func (r *RootCmd) Server(newAPI func(context.Context, *coderd.Options) (*coderd.
 				}()
 
 				options.Database = database.New(sqlDB)
-				options.Pubsub, err = pubsub.New(ctx, logger.Named("pubsub"), sqlDB, dbURL)
+				ps, err := pubsub.New(ctx, logger.Named("pubsub"), sqlDB, dbURL)
 				if err != nil {
 					return xerrors.Errorf("create pubsub: %w", err)
 				}
+				options.Pubsub = ps
+				if options.DeploymentValues.Prometheus.Enable {
+					options.PrometheusRegistry.MustRegister(ps)
+				}
 				defer options.Pubsub.Close()
 			}