feat: Add writer addr to loki canary

[yongzhang]

What this PR does / why we need it:

Which issue(s) this PR fixes:
Currently loki canary supports only one endpoint, when using it with a loki distributed cluster and push mode is enabled, there's no way to query/send logs by single endpoint.

This PR added a new flag write-addr to allow users define dedicated write endpoint to support above use case, by default, this write addr use the same value as addr.

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

[CLAassistant]

All committers have signed the CLA.

[trevorwhitney]

Can you describe your use case a little more? We usually recommend running Loki behind a gateway to have a single address for both reads and writes. Is there a reason this topology doesn't work for you?

[yongzhang]

Can you describe your use case a little more? We usually recommend running Loki behind a gateway to have a single address for both reads and writes. Is there a reason this topology doesn't work for you?

Seems like loki gateway is only introduced in Kubernetes deployment, I can't find any docs on it. If this is an optional component, probably it makes sense to allow push mode work with dedicated read/write endpoints without gateway?

[trevorwhitney]

@yongzhang yes, k8s is our most common deployment, but for OSS deployments we're just using an nginx reverse proxy in front of the cluster. You could borrow the nginx file from our helm chart and set something similar up for however you're deploying.

There are already so many config in Promtail/Loki/etc., I'd love to avoid another flag if possible 😄 , wdyt?

[yongzhang]

@yongzhang yes, k8s is our most common deployment, but for OSS deployments we're just using an nginx reverse proxy in front of the cluster. You could borrow the nginx file from our helm chart and set something similar up for however you're deploying.

There are already so many config in Promtail/Loki/etc., I'd love to avoid another flag if possible 😄 , wdyt?

ok I can try, thanks.