Based on the repository's commit message style and the changes in the diff, here's an appropriate commit message:

```
feat: add websocket tests, PoA metrics, marketplace endpoints, and enhanced observability

- Add comprehensive websocket tests for blocks and transactions streams including multi-subscriber and high-volume scenarios
- Extend PoA consensus with per-proposer block metrics and rotation tracking
- Add latest block interval gauge and RPC error spike alerting
- Enhance mock coordinator

apps/blockchain-node/src/aitbc_chain/observability/exporters.py

@@ -0,0 +1,17 @@
+"""Placeholder exporter registration for metrics/log sinks."""
+
+from __future__ import annotations
+
+from typing import Iterable
+
+REGISTERED_EXPORTERS: list[str] = []
+
+
+def register_exporters(exporters: Iterable[str]) -> None:
+    """Attach exporters for observability pipelines.
+
+    Real implementations might wire Prometheus registrations, log shippers,
+    or tracing exporters. For now, we simply record the names to keep track
+    of requested sinks.
+    """
+    REGISTERED_EXPORTERS.extend(exporters)