Fix wrong label and context for create_mr snowplow event

What does this MR do and why?

Related to #386261 (closed).

We're moving counters from Service Ping to Snowplow. i_code_review_user_create_mr is one counter we want to move.

There is already an existing Snowplow event, which was set up to mirror i_code_review_create_mr (without the user). The difference is that i_code_review_create_mr counts all the created MRs whereas i_code_review_user_create_mr counts unique users who created an MR in the timeframe.

However, our data warehouse is currently only instrumented to mirror events that count unique users, as explained in #386261 (comment 1247660020). Consequently, this MR changes the existing Snowplow instrumentation to send the correct data.

How to set up and validate locally

1. Enable snowplow micro
2. Create an MR
3. Check if a new event with i_code_review_user_create_mr action has been registered at localhost:9090/micro/good

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