Merge ticket 051: allow deleting a declared metric

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/lib/api.ts

@@ -305,6 +305,10 @@ export function updateMetric(existingName: string, request: MetricDeclaration):
   return putJson(`/api/metrics/${encodeURIComponent(existingName)}`, request)
 }
 
+export function deleteMetric(name: string): Promise<void> {
+  return sendJson(`/api/metrics/${encodeURIComponent(name)}`, 'DELETE')
+}
+
 export function fetchMetricBucketValues(
   name: string,
   params: {