[RAC] [Metrics UI] Register Metrics UI rule types with new RAC rules registry

[jasonrhodes]

⚠️ This is a meta issue.

Continuing on from the discussion in #97826, we're going to handle registering rule types separately for each remaining observability app, based on the example work done in APM. This should allow alerts created from these registered rules to begin showing up in the observability alerting table.

AC:

• Metrics rule registry is created as a child of the observability registry (see: APM example)
• Metrics rule types are registered using this new registry (see: APM example)
• Metrics rule registry is created as a child of the observability registry (see: Ifra Logs example)
• Metrics rule types are registered using this new registry (see: Ifra Logs example)

Task breakdown

• Register Inventory metric threshold as lifecycle rule #105746
• [Metrics] Register metric threshold rule type with new RAC rules registry #106222
• [RAC] Index the value and threshold for Metrics rule types when only one condition is set #106813
• [RAC][Metrics UI] Format reason in inventory metric executor #106513

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)

[mgiota]

All sub tickets are closed. This issue is fixed