Add custom context to issue activity metrics

What does this MR do and why?

Related to #362280 (closed).

These metrics have already been migrated to Snowplow, but were missing the custom context for Redis metrics migrated to Snowplow events. See the new schema for details.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Set up Snowplow Micro.
2. Do one of the instrumented issue activities, like creating an issue, to fire a Snowplow event and observe it in Snowplow Micro at http://localhost:9091/micro/good or other configured URL.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.