Cap the version of prometheus_client to <v0.9.0 in the dockerfile (#8767)

Short-term fix for https://github.com/matrix-org/synapse/issues/8766.

changelog.d/8767.bugfix

+Fix a dependency versioning bug in the Dockerfile that prevented Synapse from starting.

docker/Dockerfile

@@ -36,7 +36,8 @@ RUN pip install --prefix="/install" --no-warn-script-location \
         frozendict \
         jaeger-client \
         opentracing \
-        prometheus-client \
+        # Match the version constraints of Synapse
+        "prometheus_client>=0.4.0,<0.9.0" \
         psycopg2 \
         pycparser \
         pyrsistent \

synapse/python_dependencies.py

@@ -72,6 +72,10 @@ REQUIREMENTS = [
     # 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",
     # 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