feat: enable Prometheus metrics for ClickHouse and tenant servers

ClickHouse: enable built-in Prometheus exporter at :9363/metrics via
config.d/prometheus.xml with metrics, events, and async_metrics.
Docker labels added for docker_sd_configs auto-discovery.

Tenant servers: add prometheus.scrape/path/port labels to provisioned
server containers pointing to /api/v1/prometheus:8081.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/clickhouse-config.xml

@@ -0,0 +1,9 @@
+<clickhouse>
+  <prometheus>
+    <endpoint>/metrics</endpoint>
+    <port>9363</port>
+    <metrics>true</metrics>
+    <events>true</events>
+    <asynchronous_metrics>true</asynchronous_metrics>
+  </prometheus>
+</clickhouse>