Fix gzip content encoding quality negotiation

Honor q=0 when clients explicitly reject gzip encoding.
Add WSGI regression cases for gzip quality parameters.

Signed-off-by: Metin Hatipogullari <metin.hatipogullari@gmail.com>

Author: mhatipog

prometheus_client/exposition.py

@@ -429,8 +429,18 @@ def _get_escaping(accept_header: List[str]) -> str:
 def gzip_accepted(accept_encoding_header: str) -> bool:
     accept_encoding_header = accept_encoding_header or ''
     for accepted in accept_encoding_header.split(','):
-        if accepted.split(';')[0].strip().lower() == 'gzip':
-            return True
+        encoding, *params = accepted.split(';')
+        if encoding.strip().lower() != 'gzip':
+            continue
+        for param in params:
+            key, sep, value = param.strip().partition('=')
+            if sep and key.strip().lower() == 'q':
+                try:
+                    quality = float(value.strip())
+                except ValueError:
+                    return False
+                return 0 < quality <= 1
+        return True
     return False
 
 

tests/test_wsgi.py

@@ -112,3 +112,19 @@ def test_gzip_disabled(self):
         outputs = app(gzip_environ, self.capture)
         # Assert outputs are not compressed.
         self.assert_outputs(outputs, metric_name, help_text, increments, compressed=False)
+
+    def test_gzip_quality(self):
+        self.increment_metrics("counter", "A counter", 2)
+        app = make_wsgi_app(self.registry)
+        for encoding, compressed in [
+            ("gzip;q=0", False),
+            ("gzip;q=0.0", False),
+            ("br, GZip; Q=0", False),
+            ("gzip;q=0.5", True),
+            ("gzip;q=bogus", False),
+        ]:
+            with self.subTest(encoding=encoding):
+                environ = dict(self.environ)
+                environ['HTTP_ACCEPT_ENCODING'] = encoding
+                outputs = app(environ, self.capture)
+                self.assert_outputs(outputs, "counter", "A counter", 2, compressed)