feat(observability): add Grafana dashboard for the object data cache (#4761)

The GET body cache exports 11 `rustfs_object_data_cache_*` metrics but no
bundled dashboard visualized them. This adds one so operators can see the
cache's behaviour without hand-writing PromQL.

New `grafana-object-data-cache.json` (auto-provisioned from the dashboards
directory, `${DS_PROMETHEUS}`, schema 38) with 19 panels covering every metric:
hit ratio and lookup outcomes, plan decisions and cacheable ratio, fill
outcomes and fill-duration quantiles, hit-vs-fill byte throughput, entries and
weighted bytes vs capacity, in-flight fills, memory-pressure skips,
invalidations by reason/outcome, and size-class breakdowns. PromQL matches each
metric's type — rate() for counters, histogram_quantile() for the fill-duration
histogram, direct reads for gauges.

The observability README (EN + ZH) dashboard table gains a matching row.

Refs: backlog#1107

Co-authored-by: heihutu <heihutu@gmail.com>

.docker/observability/README.md

@@ -45,6 +45,7 @@ Three pre-built Grafana dashboards are included for monitoring RustFS GET perfor
 | **GET Rollout Health** | `grafana-get-rollout-health.json` | Monitors optimization rollout: latency by reader path, early-stop hit rate, codec streaming usage, pipeline failures |
 | **GET Data Integrity** | `grafana-get-data-integrity.json` | Monitors data safety: bitrot verify failures, decode errors, short reads, shard read outcomes |
 | **GET Resource Impact** | `grafana-get-resource-impact.json` | Monitors resource usage: concurrent requests, IO queue utilization, disk permit wait, RSS trend |
+| **Object Data Cache** | `grafana-object-data-cache.json` | Monitors the GET body cache (`rustfs_object_data_cache_*`): hit ratio, lookup/plan/fill outcomes, fill duration quantiles, hit vs fill throughput, entries/weighted bytes, inflight fills, memory-pressure skips, invalidations, and size-class breakdowns |
 
 ### Prometheus Alert Rules