Create usage metrics sub-issues for #66

[jpelletier1]

Description

Create the four requested sub-issues for the Usage Metrics PRD and link them from the parent issue.

Closes #66

Helm Chart Checklist

• I have updated the version field in Chart.yaml for each modified chart (not applicable; no Helm charts were modified)
• I have tested the chart upgrade path from the previous version (not applicable; no Helm charts were modified)
• I have verified backwards compatibility with existing values.yaml configurations (not applicable; no Helm charts were modified)
• I have updated the chart's README.md if there are any breaking changes or new required values (not applicable; no Helm charts were modified)

Additional Notes

• This PR intentionally contains no source-code changes because the requested work was GitHub issue management.
• Created and linked sub-issues [PRD] Usage Metrics: High Level Report #483, [PRD] Usage Metrics: Conversation Report #484, [PRD] Usage Metrics: User Usage Report #485, and [PRD] Usage Metrics: Model Usage Report #486.
• Updated issue [PRD] Usage Metrics #66 with a sub-issues checklist and a confirmation comment.