Emitting user tag for tidb metrics

## Feature Request

**Is your feature request related to a problem? Please describe:**

**Describe the feature you'd like:**

For query related metrics, we would like to add a `user` tag to it so that we can group the metrics by users.

**Describe alternatives you've considered:**

**Teachability, Documentation, Adoption, Migration Strategy:**

This is useful for a multi-tenant cluster, where we want to group the metrics by different users that connect to it.
Another usage is that we can define user specific alerts if different users have different SLO.