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()
  ...

```

pkg/services/pluginsintegration/clientmiddleware/metrics_middleware.go

@@ -75,10 +75,13 @@ func newMetricsMiddleware(promRegisterer prometheus.Registerer, pluginRegistry r
 
 // NewMetricsMiddleware returns a new MetricsMiddleware.
 func NewMetricsMiddleware(promRegisterer prometheus.Registerer, pluginRegistry registry.Service) backend.HandlerMiddleware {
-	imw := newMetricsMiddleware(promRegisterer, pluginRegistry)
+	metrics := newMetricsMiddleware(promRegisterer, pluginRegistry)
 	return backend.HandlerMiddlewareFunc(func(next backend.Handler) backend.Handler {
-		imw.BaseHandler = backend.NewBaseHandler(next)
-		return imw
+		return &MetricsMiddleware{
+			BaseHandler:    backend.NewBaseHandler(next),
+			pluginMetrics:  metrics.pluginMetrics,
+			pluginRegistry: metrics.pluginRegistry,
+		}
 	})
 }