Rename existing source code related metrics in known_events.yml

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As discussed in !41881 (comment 411935733), we have existing groupsource code metrics in known_events.yml. They are the following:

• wiki_action
• design_action
• project_action
• merge_request_action

Naming convention has been applied to usage pings using Redis HLL counters: https://docs.gitlab.com/ee/development/telemetry/usage_ping.html#adding-new-events.

This issue is for updating those metrics to use appropriate prefixes. Also, look into possibility of adding it to the source_code redis_slot added in !41881 (merged).

/cc @alinamihaila @marc_shaw