fix: reduce FetchMaxBytes to 10MB

[grobinson-grafana]

What this PR does / why we need it:

This pull request reduces FetchMaxBytes from 100MB to 10MB. This was causing OOMs when connected to multiple brokers with equal_spread, and also putting a lot of pressure on the GC to keep within GOMEMLIMIT of 512MB. Decreasing it to 10MB seems to have no negative effect on throughput having tested it in one of our dev cells.

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
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR