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.

src/StellaOps.Events.Mongo/EventWriter.cs

@@ -0,0 +1,25 @@
+using MongoDB.Bson;
+using MongoDB.Driver;
+using StellaOps.Provenance.Mongo;
+
+namespace StellaOps.Events.Mongo;
+
+public sealed class EventWriter
+{
+    private readonly IMongoCollection<BsonDocument> _events;
+
+    public EventWriter(IMongoDatabase db, string collectionName = "events")
+    {
+        _events = db.GetCollection<BsonDocument>(collectionName);
+    }
+
+    public async Task AppendEventAsync(
+        BsonDocument eventDoc,
+        DsseProvenance dsse,
+        TrustInfo trust,
+        CancellationToken ct = default)
+    {
+        eventDoc.AttachDsseProvenance(dsse, trust);
+        await _events.InsertOneAsync(eventDoc, cancellationToken: ct);
+    }
+}