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_ZH.md

@@ -45,6 +45,7 @@
 | **GET 发布健康度** | `grafana-get-rollout-health.json` | 监控优化发布：按 reader path 的延迟、early-stop 命中率、codec streaming 使用率、pipeline 失败率 |
 | **GET 数据完整性** | `grafana-get-data-integrity.json` | 监控数据安全：bitrot 校验失败、decode 错误、short read、shard 读取结果 |
 | **GET 资源影响** | `grafana-get-resource-impact.json` | 监控资源使用：并发请求数、IO 队列利用率、disk permit 等待、RSS 趋势 |
+| **对象数据缓存** | `grafana-object-data-cache.json` | 监控 GET body 缓存（`rustfs_object_data_cache_*`）：命中率、查找/规划/填充结果、填充耗时分位、命中 vs 填充吞吐、条目数/加权字节、在途填充、内存压力拒绝、失效、按尺寸档拆分 |
 
 ### Prometheus 告警规则