improve regexp performance in loki.process: call fmt only if debug …

[r0ka]

PR Description

Which issue(s) this PR fixes

improve regexp performance in loki.process: call fmt only if debug is enabled

Notes to the Reviewer

Was doing promtail vs alloy perf tests, and found that alloy consumes at least 40% more CPU than promtail.
here is the link for cpu.pprof
cpu pprof

PR Checklist

• [ X] CHANGELOG.md updated

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