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