Add Slack app installations to Service Ping

What does this MR do and why?

Track the number of Slack app installations, as well as the number of installations that have upgraded to the new "GBP" version.

Based on these two metrics we can calculate the progress of the update rollout.

Issue: #356000 (closed), part of &7729 (closed)

DB query plans

• CountSlackAppInstallationsMetric: https://console.postgres.ai/shared/9e8b98c3-b5c8-4969-a5ef-6ec4c9148d07
• CountSlackAppInstallationsGbpMetric (using the new index): https://console.postgres.ai/shared/7199d2fc-c245-46f1-acac-bc2b85e9aab6

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.