Move cortex metrics into tempo namespace

[obitech]

Is your feature request related to a problem? Please describe. A lot of metrics exposed by tempo are in the cortex_ namespace, for example cortex_kv_request_duration_seconds_bucket which tracks requests for the tempo-querier. This makes it confusing when building dashboards and alerts because the function is not immediately clear.

Describe the solution you'd like Move those metrics into a namespace that is unique to Tempo.

Describe alternatives you've considered Use the existing metrics.