Remove version pin prometheus_client dependency (#8875)

This removes the version pin of the `prometheus_client` dependency, in direct response to #8831. If merged, this will close #8831 

As far as I can tell, no other changes are needed, but as I'm no synapse expert, I'm relying heavily on CI and maintainer reviews for this. My very primitive test of synapse with prometheus_client v0.9.0 on my home server didn't bring up any issues, so we'll see what happens.

Signed-off-by: Jordan Bancino

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.)