Add observability to Markdown blob embeds

What does this MR do and why?

Adds observability to Render blob line of code in GFM (#300855 - closed) (FF rollout at [FF] `blob_permalink_embed` -- embed snippets o... (#623019 - closed)): metrics about renders and how those renders are going.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.