Messaging metrics (outbox + inbox) and a clear no-transport startup error

[Vulthil]

Two messaging-core improvements on one branch (off the merged main).

Observability — metrics (main change)

Mirrors the existing tracing setup, for metrics:

• Outbox — Meter "Vulthil.SharedKernel.Outbox" with vulthil.outbox.relayed / vulthil.outbox.failed counters (in OutboxProcessor); EnableMetrics flag on OutboxProcessingOptions (default on); auto-registered by AddOutboxEngine; manual AddVulthilOutboxInstrumentation() on a MeterProviderBuilder.
• Inbox — Meter "Vulthil.Messaging.Inbox" with vulthil.inbox.processed / duplicate_skipped / missing_key counters (in IdempotentConsumeFilter); EnableMetrics on InboxOptions; auto-registered by AddRelationalInbox/AddCosmosInbox; manual AddVulthilInboxInstrumentation().

Deferred (need a store-level count / terminal-marking hook): the outbox pending backlog gauge and a dead_lettered counter.

Clear startup error when no transport is configured

ConsumerHostedService now throws a clear "No messaging transport is registered — call .UseRabbitMq(...)…" at host-start instead of an obscure DI failure. Placed at host-start (not inside AddMessaging) so ReplaceTransportWithTestHarness, which registers the transport afterwards, still works.

Tests

MeterListener tests for both meters and a no-transport startup-error test; full unit suites green locally (net9.0 + net10.0).

🤖 Generated with Claude Code