Implement ledger metrics for observability and add tests for Ruby packages endpoints

- Added `LedgerMetrics` class to record write latency and total events for ledger operations.
- Created comprehensive tests for Ruby packages endpoints, covering scenarios for missing inventory, successful retrieval, and identifier handling.
- Introduced `TestSurfaceSecretsScope` for managing environment variables during tests.
- Developed `ProvenanceMongoExtensions` for attaching DSSE provenance and trust information to event documents.
- Implemented `EventProvenanceWriter` and `EventWriter` classes for managing event provenance in MongoDB.
- Established MongoDB indexes for efficient querying of events based on provenance and trust.
- Added models and JSON parsing logic for DSSE provenance and trust information.

offline/notifier/templates/attestation/tmpl-attest-key-rotation.email.en-us.template.json

@@ -0,0 +1,16 @@
+{
+  "schemaVersion": "notify.template@1",
+  "templateId": "tmpl-attest-key-rotation-email-en-us",
+  "tenantId": "bootstrap",
+  "channelType": "email",
+  "key": "tmpl-attest-key-rotation",
+  "locale": "en-us",
+  "renderMode": "html",
+  "format": "email",
+  "description": "Email bulletin for attestation key rotation or revocation events.",
+  "body": "<h2>Attestation key rotation notice</h2>\n<p>Authority rotated or revoked signing keys at {{payload.rotation.executedAt}}.</p>\n<ul>\n  <li>Rotation batch: {{payload.rotation.batchId}}</li>\n  <li>Impacted services: {{payload.rotation.impactedServices}}</li>\n  <li>Reason: {{payload.rotation.reason}}</li>\n</ul>\n<p>Recommended action: {{payload.recommendation}}</p>\n<p>Docs: <a href=\"{{payload.links.docs}}\">Rotation playbook</a></p>\n",
+  "metadata": {
+    "author": "notifications-bootstrap",
+    "version": "2025-11-12"
+  }
+}