Datasources: Set response size metric based on actual bytes read (#40303) (#40867)

Fixes #33372

(cherry picked from commit 889d4683a1)

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>

pkg/infra/httpclient/count_bytes_reader.go

@@ -0,0 +1,39 @@
+package httpclient
+
+import (
+	"io"
+)
+
+type CloseCallbackFunc func(bytesRead int64)
+
+// CountBytesReader counts the total amount of bytes read from the underlying reader.
+//
+// The provided callback func will be called before the underlying reader is closed.
+func CountBytesReader(reader io.ReadCloser, callback CloseCallbackFunc) io.ReadCloser {
+	if reader == nil {
+		panic("reader cannot be nil")
+	}
+
+	if callback == nil {
+		panic("callback cannot be nil")
+	}
+
+	return &countBytesReader{reader: reader, callback: callback}
+}
+
+type countBytesReader struct {
+	reader   io.ReadCloser
+	callback CloseCallbackFunc
+	counter  int64
+}
+
+func (r *countBytesReader) Read(p []byte) (int, error) {
+	n, err := r.reader.Read(p)
+	r.counter += int64(n)
+	return n, err
+}
+
+func (r *countBytesReader) Close() error {
+	r.callback(r.counter)
+	return r.reader.Close()
+}