Add tracking for PE features

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Release notes

Problem to solve

As a GitLab Product Manager for PE, I want to see metrics about the group's features, so we can make data informed decisions.

Proposal

From the comment in Product Analytics project.

Add Snowplow or Service Ping metrics via backend product code for

1. merge train usage
2. merge results pipelines
3. external_pipelines
4. Secure ci_job_token usage - Broken out to #366378 (closed)

For 1, 2, 3, this should be 'count of users who have run a pipeline. For 4 'count of projects with a ci_job_token enabled'

Intended users

Feature Usage Metrics