test: add coalescer delay cost report (#6464)

Add a read-only Prometheus report helper for backlog#2007 so the 200us vs 50us coalescer delay experiment can capture RPC, batch distribution, stage latency, and host-cost signals with one fixed evidence format.

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

docs/testing/issue-2007-coalescer-delay.md

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+# Backlog #2007 Coalescer Delay Validation
+
+`scripts/issue_2007_coalescer_prometheus_report.py` is a read-only Prometheus
+report helper for validating whether the GET metadata `ReadVersion` coalescer
+default can move from `200us` to `50us`.
+
+The benchmark itself is intentionally external to this helper: use the same
+main build, bucket/object set, workload, and
+`RUSTFS_BATCH_READ_VERSION_SERVER_PARALLELISM=4` for both cells. Only switch:
+
+```bash
+RUSTFS_GET_METADATA_READ_VERSION_COALESCE=auto
+RUSTFS_GET_METADATA_READ_VERSION_COALESCE_DELAY_MICROS=200
+RUSTFS_GET_METADATA_READ_VERSION_COALESCE_DELAY_MICROS=50
+```
+
+After each measured workload window, collect a report from Prometheus:
+
+```bash
+scripts/issue_2007_coalescer_prometheus_report.py \
+  --query-url http://prometheus.example:9090 \
+  --profile delay-200us \
+  --window 180s \
+  --rustfs-selector 'server=~"node[5-8]"' \
+  --node-selector 'instance=~"node[5-8].*"'
+
+scripts/issue_2007_coalescer_prometheus_report.py \
+  --query-url http://prometheus.example:9090 \
+  --profile delay-50us \
+  --window 180s \
+  --rustfs-selector 'server=~"node[5-8]"' \
+  --node-selector 'instance=~"node[5-8].*"'
+```
+
+The output is Markdown and is suitable for attaching to the issue alongside the
+warp throughput, average latency, p95, p99, and TTFB p99 from the fixed
+workload run.
+
+Required RustFS signals:
+
+- `grpc_read_version` and `grpc_batch_read_version` outgoing request increases.
+- Coalescer batch distribution from
+  `rustfs_get_metadata_read_version_coalescer_total{event="attempted_batch"}`.
+- `batch_read_version_coalescer_wait`, `batch_read_version_rpc_roundtrip`,
+  `batch_read_version_disk_read`, and `batch_read_version_response_map` p99.
+
+Required host-cost signals:
+
+- CPU busy from `node_cpu_seconds_total`.
+- Network RX/TX from `node_network_receive_bytes_total` and
+  `node_network_transmit_bytes_total`.
+- Disk read await, average queue depth, and utilization from node-exporter disk
+  counters.
+
+If a section reports `UNAVAILABLE`, treat that evidence as missing rather than
+zero. Do not use a default-change PR until the `50us` cell has stable
+throughput/latency benefit and CPU, network, and disk cost are available and
+acceptable.