envd: avoid regex matching in Prometheus queries

[teskje]

For the queries that expose all transient collections in a single "transient" bucket, we simply need to switch on the first character of the collection ID. No need to invoke the regex machinery for that.

Motivation

This removes an inefficiency identified in Slack. Not clear if it's really noticeable, but no harm in fixing it.

[github-actions]

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[antiguru]

Thanks!

[teskje]

TFTR!