Stopped String() from doubly escaping regex characters (#10277)

**What this PR does / why we need it**:
When queries have regex, characters that needed escaping were escaped
twice. Now they're only singly escaped. This is good because it reduces
the character count so users can submit queries closer to the 5120 byte
limit.

**Which issue(s) this PR fixes**:
Sort of addresses #10153

**Special notes for your reviewer**:

**Checklist**
- [x] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [ ] Documentation added
- [x] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] If the change is worth mentioning in the release notes, add
`add-to-release-notes` label
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/setup/upgrade/_index.md`
- [ ] For Helm chart changes bump the Helm chart version in
`production/helm/loki/Chart.yaml` and update
`production/helm/loki/CHANGELOG.md` and
`production/helm/loki/README.md`. [Example
PR](d10549e3ec)

Co-authored-by: Akhila Narayanan <akhilanarayanan@Akhilas-MacBook-Air.local>

pkg/logql/log/label_filter.go

@@ -368,6 +368,16 @@ type lineFilterLabelFilter struct {
 	filter Filterer
 }
 
+// overrides the matcher.String() function in case there is a regexpFilter
+func (s *lineFilterLabelFilter) String() string {
+	if unwrappedFilter, ok := s.filter.(regexpFilter); ok {
+		rStr := unwrappedFilter.String()
+		str := fmt.Sprintf(`%s%s"%s"`, s.Matcher.Name, s.Matcher.Type, rStr)
+		return str
+	}
+	return s.Matcher.String()
+}
+
 func (s *lineFilterLabelFilter) Process(_ int64, line []byte, lbs *LabelsBuilder) ([]byte, bool) {
 	v := labelValue(s.Name, lbs)
 	return line, s.filter.Filter(unsafeGetBytes(v))

pkg/logql/syntax/parser_test.go

@@ -3465,3 +3465,13 @@ func TestNoOpLabelToString(t *testing.T) {
 	require.NoError(t, err)
 	require.Len(t, stages, 0)
 }
+
+// Tests that the regex part of expr doesn't get doubly escaped when we call expr.String()
+func TestParseSampleExpr_String(t *testing.T) {
+	query := `sum(rate({cluster="beep", namespace="boop"} | msg=~` + "`" + `.*?(GET|POST|PUT|DELETE|PATCH|HEAD|OPTIONS) /loki/api/(?i)(\d+[a-z]|[a-z]+\d)\w*/query_range` + "`" + `[1d]))`
+	expr, err := ParseSampleExpr(query)
+	require.NoError(t, err)
+	// need to change some backticks to " in order for query to be exactly equal to expr.String(), otherwise it's the same query
+	expected := `sum(rate({cluster="beep", namespace="boop"} | msg=~".*?(GET|POST|PUT|DELETE|PATCH|HEAD|OPTIONS) /loki/api/(?i)(\d+[a-z]|[a-z]+\d)\w*/query_range"[1d]))`
+	require.Equal(t, expected, expr.String())
+}