feat: change loki_kafka_reader_receive_delay_seconds to gauge

[grobinson-grafana]

What this PR does / why we need it:

This commit changes loki_kafka_reader_receive_delay_seconds from a histogram to a gauge. It doesn't make sense to plot this as a histogram, it is a single dimension stat that we want to track as it increases and decreases.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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