fix(helm) Add capability to ignore the gateway service by the serviceMonitor

[winem]

What this PR does / why we need it:
This PR sets per default the label prometheus.io/scrape=false to the gateway service.

This causes the gateway service not to be treated as a Prometheus scrape target and fixes alerts that the target is down and can't be scraped. This is useful because the nginx does not provide any metrics in the default configuration.

Users could either add the label manually or we set it per default. With this PR I would pitch the 2nd option so that users don't have to investigate the alerts caused by a default loki deployment via helm.

Which issue(s) this PR fixes:
Could not find any related issue and experienced the issue by myself when switching from the loki-distributed to the loki chart.

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
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• 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

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