Prometheus: Remove old client

[aocenas]

Recently a new client was added to the Prometheus plugin to simplify response processing and improve performance and memory consumption. We still did not remove the old client and before we can do that there are a few things:

• Put the old client behind the feature flag and make new client default
• Observe if there are any issues
• Remove old client code totally
• Remove comparison logic

#52738 (comment)

[evandandrea]

@itsmylife to sync up with Beto, Kyle this week. Kyle has started to look into it, but hasn't gotten exemplars working.

[gtk-grafana]

@itsmylife Create epic please!

[itsmylife]

This is a task for the epic #52926

[bohandley]

As of the plan put forward today put forward by Kyle,

• v9.2: New client flag is there, should be feature complete, we encourage people to use it
• v9.3: New client is default, flag to use old. Old client deprecated, no changes
• v9.4: Delete old client (Feb)

https://raintank-corp.slack.com/archives/C03FZ9T50R1/p1665073148493249

We are moving this out of Q3 and potentially Q1 of 2023

[bohandley]

Closing this to reorganize into this issue: https://github.com/grafana/observability-metrics-squad/issues/53