`record_uring_fallback` is called at multiple sites in the io_uring read
path whenever a read falls back to `StdBackend`. It formatted
`self.root.display().to_string()` on every call, heap-allocating a
`String` from a `Path` that never changes after construction — pure
per-read waste when io_uring is degraded.
Cache the label once in `UringBackend::try_new` as a `String` field
(`root_label`) and clone it per emission. The metric name and the
`"root"` label value are unchanged; only the redundant `Path` formatting
is removed. The clone is a single alloc of an already-short string.
Refs: rustfs/backlog#1185
Co-authored-by: heihutu <heihutu@gmail.com>
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