Feature Request: Add path/subpath parameter to loki output plugin

[PeterStolz]

Is your feature request related to a problem? Please describe.
I use a managed instance of a grafana+loki stack from my cloud provider. Instead of giving me a subdomain for my instance I need to specify a subpath e.g. /instances/my-instance-id/ before the loki/api/v1/push. This is not supported by the current plugin as there is no path parameter and the plugin does a direct DNS lookup on the host part. Therefore I can not ingest logs with official plugins.

Describe the solution you'd like
Add a path or path_prefix parameter allowing easier integration of loki instances behind reverse proxies.

Describe alternatives you've considered
Grafana has their own loki plugin (grafana-loki), which is able to handle custom paths. The problem is that they deprecated their helm chart for fluent bit that ships with this plugin. Installing plugins in the official fluent bit helm chart is not streamlined.

Additional context
I now either need to hack the grafana-loki plugin into the helm chart or deploy a reverse proxy specifically to add a path prefix.

The code change would be fairly simple and straight forward as there is already code in place that builds the URL from the http basic auth, the hostnam, the port and the standard loki/api/v1/push.

If this feature would be accepted I will create a PR for it.
Thanks for your consideration.