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/Findings/StellaOps.Findings.Ledger/Services/LedgerEventWriteService.cs

@@ -1,8 +1,10 @@
+using System.Diagnostics;
 using System.Text.Json.Nodes;
 using Microsoft.Extensions.Logging;
 using StellaOps.Findings.Ledger.Domain;
 using StellaOps.Findings.Ledger.Hashing;
 using StellaOps.Findings.Ledger.Infrastructure;
+using StellaOps.Findings.Ledger.Observability;
 
 namespace StellaOps.Findings.Ledger.Services;
 
@@ -29,6 +31,8 @@ public sealed class LedgerEventWriteService : ILedgerEventWriteService
 
     public async Task<LedgerWriteResult> AppendAsync(LedgerEventDraft draft, CancellationToken cancellationToken)
     {
+        var stopwatch = Stopwatch.StartNew();
+
         var validationErrors = ValidateDraft(draft);
         if (validationErrors.Count > 0)
         {
@@ -95,6 +99,9 @@ public sealed class LedgerEventWriteService : ILedgerEventWriteService
         {
             await _repository.AppendAsync(record, cancellationToken).ConfigureAwait(false);
             await _merkleAnchorScheduler.EnqueueAsync(record, cancellationToken).ConfigureAwait(false);
+
+            stopwatch.Stop();
+            LedgerMetrics.RecordWriteSuccess(stopwatch.Elapsed, draft.TenantId, draft.EventType, DetermineSource(draft));
         }
         catch (Exception ex) when (IsDuplicateKeyException(ex))
         {
@@ -116,6 +123,21 @@ public sealed class LedgerEventWriteService : ILedgerEventWriteService
         return LedgerWriteResult.Success(record);
     }
 
+    private static string DetermineSource(LedgerEventDraft draft)
+    {
+        if (draft.SourceRunId.HasValue)
+        {
+            return "policy_run";
+        }
+
+        return draft.ActorType switch
+        {
+            "operator" => "workflow",
+            "integration" => "integration",
+            _ => "system"
+        };
+    }
+
     private static bool IsDuplicateKeyException(Exception exception)
     {
         if (exception is null)