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

Fixes #33372

pkg/infra/httpclient/count_bytes_reader_test.go

@@ -0,0 +1,38 @@
+package httpclient
+
+import (
+	"fmt"
+	"io/ioutil"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestCountBytesReader(t *testing.T) {
+	tcs := []struct {
+		body               string
+		expectedBytesCount int64
+	}{
+		{body: "d", expectedBytesCount: 1},
+		{body: "dummy", expectedBytesCount: 5},
+	}
+
+	for index, tc := range tcs {
+		t.Run(fmt.Sprintf("Test CountBytesReader %d", index), func(t *testing.T) {
+			body := ioutil.NopCloser(strings.NewReader(tc.body))
+			var actualBytesRead int64
+
+			readCloser := CountBytesReader(body, func(bytesRead int64) {
+				actualBytesRead = bytesRead
+			})
+
+			bodyBytes, err := ioutil.ReadAll(readCloser)
+			require.NoError(t, err)
+			err = readCloser.Close()
+			require.NoError(t, err)
+			require.Equal(t, tc.expectedBytesCount, actualBytesRead)
+			require.Equal(t, string(bodyBytes), tc.body)
+		})
+	}
+}