Datasources: Introduce response_limit for datasource responses (#38962)

* Introduce response_limit for datasource responses

* Fix lint

* Fix tests

* Add case where limit <= 0 - added parametrized tests

* Add max_bytes_reader.go

* Use new httpclient.MaxBytesReader instead of net/http one

* Fixes according to reviewer's comments

* Add tests for max_bytes_reader

* Add small piece in configuration.md

* Further fixes according to reviewer's comments

* Fix linting - fix test

pkg/infra/httpclient/max_bytes_reader_test.go

@@ -0,0 +1,40 @@
+package httpclient
+
+import (
+	"errors"
+	"fmt"
+	"io/ioutil"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestMaxBytesReader(t *testing.T) {
+	tcs := []struct {
+		limit      int64
+		bodyLength int
+		body       string
+		err        error
+	}{
+		{limit: 1, bodyLength: 1, body: "d", err: errors.New("error: http: response body too large, response limit is set to: 1")},
+		{limit: 1000000, bodyLength: 5, body: "dummy", err: nil},
+		{limit: 0, bodyLength: 0, body: "", err: errors.New("error: http: response body too large, response limit is set to: 0")},
+	}
+	for _, tc := range tcs {
+		t.Run(fmt.Sprintf("Test MaxBytesReader with limit: %d", tc.limit), func(t *testing.T) {
+			body := ioutil.NopCloser(strings.NewReader("dummy"))
+			readCloser := MaxBytesReader(body, tc.limit)
+
+			bodyBytes, err := ioutil.ReadAll(readCloser)
+			if err != nil {
+				require.EqualError(t, tc.err, err.Error())
+			} else {
+				require.NoError(t, tc.err)
+			}
+
+			require.Len(t, bodyBytes, tc.bodyLength)
+			require.Equal(t, string(bodyBytes), tc.body)
+		})
+	}
+}