Parse escaped quotes in OpenMetrics exemplar label values (#1230)

The exemplar scanner in _parse_remaining_text flipped its in-quotes
state on every double quote, so a label value like x"y, which the
writer emits as x\"y, made the whole line fail with "Invalid line".
Ignore quotes preceded by an odd number of backslashes.

Signed-off-by: spicysharkco <321699413+spicysharkco@users.noreply.github.com>

Author: spicysharkco

prometheus_client/openmetrics/parser.py

@@ -154,10 +154,9 @@ def _parse_remaining_text(text):
     state = 'timestamp'
     text = split_text[1]
 
-    it = iter(text)
     in_quotes = False
-    for char in it:
-        if char == '"':
+    for i, char in enumerate(text):
+        if char == '"' and not _is_character_escaped(text, i):
             in_quotes = not in_quotes
         if in_quotes:
             continue

tests/openmetrics/test_parser.py

@@ -570,6 +570,17 @@ def test_label_escaping(self):
             metric_family.add_metric([unescaped_val, "baz"], 1)
             self.assertEqual([metric_family], list(families))
 
+    def test_exemplar_label_escaping(self):
+        for escaped_val, unescaped_val in [('\\"', '"'), ('\\\\', '\\'), ('\\\\\\"', '\\"')]:
+            families = list(text_string_to_metric_families("""# TYPE a counter
+# HELP a help
+a_total 1 # {foo="%s"} 1
+# EOF
+""" % escaped_val))
+            metric_family = CounterMetricFamily("a", "help")
+            metric_family.add_sample("a_total", {}, 1.0, None, Exemplar({"foo": unescaped_val}, 1))
+            self.assertEqual([metric_family], list(families))
+
     def test_help_escaping(self):
         for escaped_val, unescaped_val in [
                 ('foo', 'foo'),