fix: Change pushHandler to return 200 not 204

[sysadmin1139]

What this PR does / why we need it:
Changes response-code for the /otlp/v1/logs endpoint to return 200-OK instead of 204-NoContent.

See also:

• https://opentelemetry.io/docs/specs/otlp/#full-success-1 for the OTLP endpoint and what it accepts, which is only 200.
• Exporters logs error message and returns error code for successful operations with return code 204 open-telemetry/opentelemetry-python#3621 referencing Grafana Loki directly
• Response success status validated incorrectly open-telemetry/opentelemetry-python#4338 regarding Zipkin endpoints and why validations are using both 200 and 202

The 200-OK behavior is per the OTLP specification this endpoint uses.

Which issue(s) this PR fixes:
Fixes #15761

Special notes for your reviewer:
This is a change of behavior from historical, but is in better compliance with the spec.

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

[CLAassistant]

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