Improve documentation of Prometheus metrics

[ThomWright]

Report needed documentation A list of metrics Milvus exposes, including the metric names. Knowing the metric names makes it much easier to use.

Describe the documentation you'd like A table describing the Prometheus metrics exposed by Milvus. The table here shows a good example of how this could be formatted: https://prometheus.io/docs/instrumenting/writing_clientlibs/#process-metrics - notice the name and unit columns.

Steps taken to search for needed documentation The metrics are currently described here: https://milvus.io/docs/v0.10.1/setup_grafana.md

I think this should be taken out of the 'Grafana' section. The exposed metrics are not really related to Grafana (I could use something else to graph them), so at first I didn't look here. They could be in a Prometheus section, or their own section.

[fishpenguin]

We will fix the metric names and the documentation in milvus-0.11.0.

[wxyucs]

Not needed.