fix(webapp): address review on db pool metrics registry

Back the metrics-source registry with singleton() keyed by clientType, matching
the app's other process-wide registries and deduping so a re-evaluated module or
a repeated label registers once. Document the removal of prisma_* from the
Prometheus /metrics endpoint in the server-changes note.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KHjfL7qXHia5DTnxi1RePS

Author: ericallam

.server-changes/db-pool-metrics-per-client.md

@@ -3,4 +3,4 @@ area: webapp
 type: improvement
 ---
 
-Database connection-pool and query metrics are now reported for every configured database connection rather than only the primary, and keep working regardless of which database driver a connection uses.
+Database connection-pool and query metrics are now reported for every configured database connection rather than only the primary, and keep working regardless of which database driver a connection uses. These metrics are now emitted only through the OpenTelemetry exporter; the Prometheus `/metrics` endpoint no longer includes the `prisma_*` series, so if you scrape that endpoint for database metrics, enable the OpenTelemetry metric exporter instead.

apps/webapp/app/utils/databaseMetrics.server.ts

@@ -1,3 +1,5 @@
+import { singleton } from "./singleton";
+
 export type MetricHistogramValue = {
   buckets: [number, number][];
   sum: number;
@@ -50,18 +52,21 @@ export type NormalizedDatabaseMetrics = {
   };
 };
 
-const sources: DatabaseMetricsSource[] = [];
+const sources = singleton(
+  "databaseMetricsSources",
+  () => new Map<string, DatabaseMetricsSource>()
+);
 
 export function registerDatabaseMetricsSource(source: DatabaseMetricsSource): void {
-  sources.push(source);
+  sources.set(source.clientType, source);
 }
 
 export function listDatabaseMetricsSources(): ReadonlyArray<DatabaseMetricsSource> {
-  return sources;
+  return Array.from(sources.values());
 }
 
 export function resetDatabaseMetricsSources(): void {
-  sources.length = 0;
+  sources.clear();
 }
 
 function indexByKey(entries: Array<{ key: string; value: number }>): Record<string, number> {
@@ -130,7 +135,7 @@ export function normalizeDatabaseMetrics(
 
 export async function collectDatabaseClientMetrics(): Promise<NormalizedDatabaseMetrics[]> {
   return Promise.all(
-    sources.map(async (source) => {
+    Array.from(sources.values()).map(async (source) => {
       let json: PrismaMetricsJson | undefined;
       try {
         json = await source.client.$metrics.json();