`report_metrics` runs on every metrics collection cycle and did three things it
did not need to, for every metric, every cycle:
- interned `metric.name`/`metric.help` through a `Mutex<HashMap>` even when the
`Cow` was already `Borrowed(&'static str)` (the common case for statically
named metrics);
- re-ran `describe_*!` (which re-locks the recorder's metadata map) although the
metadata never changes;
- allocated a fresh `Vec<(String, String)>`, cloning every label key and value,
even though `metric.labels` is already `[(&'static str, Cow<'static, str>)]`.
Now: names/help resolve to `&'static str` without touching the intern cache when
already borrowed; each metric is described once (tracked in a `HashSet`); and the
recorder is fed `&metric.labels` directly, removing the per-cycle label clone.
No metric names, help, label keys/values, or emitted values change. Verified by
building rustfs-obs and running the report unit tests (the `metrics` macro
accepts the borrowed label slice directly).
Addresses rustfs/backlog#1185 (P3, report path).
Co-authored-by: heihutu <heihutu@gmail.com>
houseme