fix: remove space after comma in openmetrics exposition

OpenMetrics doesn't allow for spaces between labels and prometheus fails with a
parsing error.

Removing this fixes UTF8 metrics exposition

Signed-off-by: Dominik Süß <dominik@suess.wtf>

Author: theSuess

prometheus_client/openmetrics/exposition.py

@@ -72,7 +72,7 @@ def generate_latest(registry, escaping=UNDERSCORES, version="1.0.0"):
                 if escaping == ALLOWUTF8 and not _is_valid_legacy_metric_name(s.name):
                     labelstr = escape_metric_name(s.name, escaping)
                     if s.labels:
-                        labelstr += ', '
+                        labelstr += ','
                 else:
                     labelstr = ''