Fix flaky prom remote write exporter concurrency test (#37430)

Fix #37104

This is more an artifact of the test firing an unbounded number of go
routines each one making its own HTTP request. Although keepalive is
enabled by default the code ends up not re-using many of the connections
causing the many connections to end up in a TIME_WAIT state. In order to
avoid this the test now limits the number of concurrent requests and has
a small change to the actual code to facilitate re-use of existing TCP
connections used by the HTTP client.

Although there is a change to non-test code I don't consider this a bug
worth changelog because no user of the component should reach such high
burst of "push metrics" in any reasonable production scenario.

exporter/prometheusremotewriteexporter/exporter.go

@@ -353,7 +353,10 @@ func (prwe *prwExporter) execute(ctx context.Context, writeReq *prompb.WriteRequ
 		if err != nil {
 			return err
 		}
-		defer resp.Body.Close()
+		defer func() {
+			_, _ = io.Copy(io.Discard, resp.Body)
+			resp.Body.Close()
+		}()
 
 		// 2xx status code is considered a success
 		// 5xx errors are recoverable and the exporter should retry

exporter/prometheusremotewriteexporter/exporter_concurrency_test.go

@@ -8,7 +8,7 @@ import (
 	"io"
 	"net/http"
 	"net/http/httptest"
-	"os"
+	"runtime"
 	"strconv"
 	"sync"
 	"testing"
@@ -32,9 +32,6 @@ import (
 // Test everything works when there is more than one goroutine calling PushMetrics.
 // Today we only use 1 worker per exporter, but the intention of this test is to future-proof in case it changes.
 func Test_PushMetricsConcurrent(t *testing.T) {
-	if os.Getenv("ImageOs") == "win25" && os.Getenv("GITHUB_ACTIONS") == "true" {
-		t.Skip("Skipping test on Windows 2025 GH runners, see https://github.com/open-telemetry/opentelemetry-collector-contrib/issues/37104")
-	}
 	n := 1000
 	ms := make([]pmetric.Metrics, n)
 	testIDKey := "test_id"
@@ -137,15 +134,22 @@ func Test_PushMetricsConcurrent(t *testing.T) {
 		resp, checkRequestErr := http.Get(server.URL)
 		require.NoError(c, checkRequestErr)
 		assert.NoError(c, resp.Body.Close())
-	}, 5*time.Second, 100*time.Millisecond)
+	}, 15*time.Second, 100*time.Millisecond)
 
 	var wg sync.WaitGroup
 	wg.Add(n)
+	maxConcurrentGoroutines := runtime.NumCPU() * 4
+	semaphore := make(chan struct{}, maxConcurrentGoroutines)
 	for _, m := range ms {
+		semaphore <- struct{}{}
 		go func() {
+			defer func() {
+				<-semaphore
+				wg.Done()
+			}()
+
 			err := prwe.PushMetrics(ctx, m)
 			assert.NoError(t, err)
-			wg.Done()
 		}()
 	}
 	wg.Wait()