Remove version pin prometheus_client dependency

changelog.d/8875.misc

@@ -0,0 +1 @@
+Add support for the latest third-party libraries. Contributed by Jordan Bancino.

docker/Dockerfile

@@ -37,7 +37,7 @@ RUN pip install --prefix="/install" --no-warn-script-location \
         jaeger-client \
         opentracing \
         # Match the version constraints of Synapse
-        "prometheus_client>=0.4.0,<0.9.0" \
+        "prometheus_client>=0.4.0" \
         psycopg2 \
         pycparser \
         pyrsistent \

synapse/python_dependencies.py

@@ -40,6 +40,10 @@
 # Note that these both represent runtime dependencies (and the versions
 # installed are checked at runtime).
 #
+# Also note that we replicate these constraints in the Synapse Dockerfile while
+# pre-installing dependencies. If these constraints are updated here, the same
+# change should be made in the Dockerfile.
+#
 # [1] https://pip.pypa.io/en/stable/reference/pip_install/#requirement-specifiers.
 
 REQUIREMENTS = [
@@ -69,14 +73,7 @@
     "msgpack>=0.5.2",
     "phonenumbers>=8.2.0",
     # we use GaugeHistogramMetric, which was added in prom-client 0.4.0.
-    # prom-client has a history of breaking backwards compatibility between
-    # minor versions (https://github.com/prometheus/client_python/issues/317),
-    # so we also pin the minor version.
-    #
-    # Note that we replicate these constraints in the Synapse Dockerfile while
-    # pre-installing dependencies. If these constraints are updated here, the
-    # same change should be made in the Dockerfile.
-    "prometheus_client>=0.4.0,<0.9.0",
+    "prometheus_client>=0.4.0",
     # we use attr.validators.deep_iterable, which arrived in 19.1.0 (Note:
     # Fedora 31 only has 19.1, so if we want to upgrade we should wait until 33
     # is out in November.)