[New diffs] Track metrics on how much dependency linker is used

Background

This is based on #430923 (comment 1693572658).

We need some metrics to see how much dependency linker is used when syntax highlight blobs/diffs so it can help us decide what we need to do about it.

Proposal

Add metrics to get the following:

• total usage of Gitlab::Highlight
• usage of dependency linker on blobs
• usage of dependency linker on diffs

We can probably expose this metrics via Prometheus: https://docs.gitlab.com/ee/development/prometheus_metrics.html.