Give AMS its own metrics/tracing surface

[JSONbored]

Problem: The main product has a full Prometheus + OpenTelemetry + Grafana observability stack; none of it reaches the miner package, which has no metrics/tracing surface of its own beyond the one-off Prometheus renderer above.

Area: AMS / Observability

Proposal: Add a minimal, optional metrics/tracing hook to the miner CLI that a self-hoster can wire into their own observability stack, following the main product's opt-in pattern rather than requiring one.

Deliverables:

• An optional metrics/tracing integration point, documented, off by default.

Acceptance criteria:

• A self-hoster can opt in and see miner activity in their own Grafana/observability stack.

Boundaries:

• Keep this genuinely optional — AMS's zero-infra "laptop mode" use case must keep working with no observability stack present at all.

[linear-code]

JSO-3499