Add OpenTelemetry metrics for retry and archive operations

[johnsimons]

Adds telemetry for error management, the follow-up to the ingestion (#5816) and retention (#5831) metrics. Retry and archive were the last long-running operations in the error instance with no signal at all: a retry of a large group can run for hours across four stages and two polling loops, and until now the only way to see it was the log.

What is instrumented

sc.retry.* covers the whole retry pipeline: an end-to-end operation duration from request to completion, a duration histogram per stage (prepare, stage, forward), a message counter by outcome, an in-progress gauge, and the depth of the bulk request queue. The abandoned message outcome is the one worth alerting on, since it counts messages that hit the staging retry limit and were dropped from their batch, which previously existed only as a log line. The in-progress gauge is what surfaces a stuck operation, because an operation that never completes never records a duration.

sc.archive.* covers group archive and unarchive: whole-operation duration, per-batch duration, a message counter, and an in-progress gauge. The instrumentation lives once in the shared InMemoryArchive/InMemoryUnarchive state machines, but only the EF persistence registers ArchiveMetrics, so on RavenDB the family does not exist rather than being half-populated. No RavenDB file is touched.

The host also gains the standard ASP.NET Core, HTTP client and runtime instrumentation packages, behind the existing OTEL_EXPORTER_OTLP_ENDPOINT gate. That covers the read APIs per route, the scatter-gather calls to remote instances, and GC/thread pool context. AddIngestionMetrics is renamed AddTelemetry now that it wires more than ingestion.

All tag values come from enums, so series counts are bounded; group ids, endpoint names and queue addresses stay out and are recovered from the audit log when a specific operation needs investigating. The full instrument contract is documented in docs/telemetry.md.

[rbev]

Not necessarily better but you could avoid all the string constants using enum.ToString()

Suggested change

	static string ResultTag(ScopeOutcome outcome) => outcome switch
	{
	ScopeOutcome.Success => "success",
	ScopeOutcome.Empty => "empty",
	ScopeOutcome.Cancelled => "cancelled",
	ScopeOutcome.Failed => "failed",
	_ => "failed"
	};
	static string ResultTag(ScopeOutcome outcome) => (Enum.IsDefined(outcome) ? outcome : ScopeOutcome.Failed).ToString().ToLower();