Add Prometheus metrics to the boundary log proxy for observability:
- batches_dropped_total (reason: buffer_full, forward_failed)
- logs_dropped_total (reason: buffer_full, forward_failed,
boundary_channel_full, boundary_batch_full)
- batches_forwarded_total
Also add BoundaryStatus to the BoundaryMessage envelope so boundary
can report dropped log counts as a separate wire message. The agent
records these as Prometheus metrics, making boundary-side data loss
visible. Backwards compatibility for older versions of boundary is maintained.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>