Make loki.LogsReceiver an interface

[thampiotr]

PR Description

In order to implement config converter we need to override how a components' export is tokenized.

• There is an example of how this is done here for prometheus.remote_write - when we create a block for remote_write, we return an remotewrite.Exports.
• That export is then set on river components args that use it and since we override the way its tokenized, we control how it is displayed in the final River configuration that is generated. In the example above, the exported receiver will be displayed as "prometheus.remote_write.default.receiver" which is what we want.
• However, the trick to override how an export value is tokenized requires us to extend the exported type, which can only be done when it's an interface.
• loki.LogsReceiver is not an interface, so I ran into an issue when trying to implement promtail config conversion support. The problem can be seen in code in this draft PR.

Proposed solution: make loki.LogsReceiver an interface.

PR Checklist

• CHANGELOG updated
• Documentation added
• Tests updated

[thampiotr]

This is the main change - the rest is just making it work using it.

[mattdurham]

Might be best for a separate PR, but I am wondering if its possible to hide the underlying implementation and the interface be more generic while we are looking at it.

type LogsReceiver interface {
	Send(Entry)
        Receive() Entry
}

[mattdurham]

LGTM

[erikbaranowski]

LGTM

[thampiotr]

Might be best for a separate PR, but I am wondering if its possible to hide the underlying implementation and the interface be more generic while we are looking at it.

type LogsReceiver interface {
	Send(Entry)
        Receive() Entry
}

There's also some places where close(channel) is called, so not 100% sure we can hide that it's a channel and hence it may not be useful to do that.