grpc: Switch to go-grpc-middleware/providers/prometheus

[pgporada]

While investigating #6384, I noticed that go-grpc-prometheus was deprecated last year and users should switch to go-grpc-middleware instead. The default prometheus histogram buckets will continue to be used and can be found here.

[beautifulentropy]

Ideally these two changes would be broken into two PRs but I think this is probably fine as well.

[pgporada]

I see what you're saying, like make the switch using whatever the default buckets are and then the followup change would have been adding additional buckets. I considered that but thought it was such an inconsequential change and tacked it on here.