Plugin Metrics: Eliminate data race in plugin metrics middleware (#99396)
fix: eliminate data race in plugin metrics middleware
A data race was detected when multiple goroutines accessed the `MetricsMiddleware`
simultaneously. The race occurred because a single `MetricsMiddleware` instance
was being shared across goroutines while its `BaseHandler` field was being
modified during middleware chain setup.
Fix by creating a new `MetricsMiddleware` instance for each middleware chain,
while safely sharing the thread-safe Prometheus metrics and plugin registry.
This maintains proper metrics collection while eliminating the mutable shared
state that caused the race condition.
Original error was detected here:
```
WARNING: DATA RACE
Read at 0x00c0039c0790 by goroutine 4486:
github.com/grafana/grafana-plugin-sdk-go/backend.(*ErrorSourceMiddleware).CallResource()
/Users/clord/src/grafana/irm-devstack/.devenv/state/go/pkg/mod/github.com/grafana/grafana-plugin-sdk-go@v0.261.0/backend/error_source_middleware.go:93 +0x40
github.com/grafana/grafana-plugin-sdk-go/backend.BaseHandler.CallResource()
...
```
(cherry picked from commit e74cf72d99)
Co-authored-by: Christopher Lord <christopher.lord@grafana.com>
f99b152dc1