Add comment clarifying cache behavior of `GrafanaMetricsEmbedService`

What does this MR do?

Follow-up from Add service to fetch and format a grafana charts embed.

Adds a comment explaining the kind of strange code style in GrafanaMetricsEmbedService.

Closes #34526 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [n/a] Changelog entry
• [n/a] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [n/a] Database guides
• [n/a] Separation of EE specific content