Performance: add preallocation for some slice/map (#57860)

This change preallocates slices and maps where the size of the data is known before the object is created.

Co-authored-by: Joe Blubaugh <joe.blubaugh@grafana.com>

pkg/tsdb/prometheus/querydata/request.go

@@ -185,7 +185,7 @@ func (s *QueryData) trace(ctx context.Context, q *models.Query) (context.Context
 }
 
 func sdkHeaderToHttpHeader(headers map[string]string) http.Header {
-	httpHeader := make(http.Header)
+	httpHeader := make(http.Header, len(headers))
 	for key, val := range headers {
 		httpHeader[key] = []string{val}
 	}