[exporter/loki] add support for logfmt

[domasx2]

Is your feature request related to a problem? Please describe.

I have a use case for sending logfmt-formatted log records through the otelcol to Loki. But loki exporter encodes them in JSON, ending up with { "Body": "{{message}}" }. This requires workarounds down the line to unwrap the JSON.
It would be great to have direct logfmt support for the Loki exporter.

Describe the solution you'd like

Add a loki.format attribute hint, that, if provided with value logfmt, will cause exporter to format logs using logfmt instead of JSON.

Describe alternatives you've considered

No response

Additional context

No response

[github-actions]

This issue has been inactive for 60 days. It will be closed in 60 days if there is no activity. To ping code owners by adding a component label, see Adding Labels via Comments, or if you are unsure of which component this issue relates to, please ping @open-telemetry/collector-contrib-triagers. If this issue is still relevant, please ping the code owners or leave a comment explaining why it is still relevant. Otherwise, please close it.

Pinging code owners:

• exporter/loki: @gramidt @gouthamve @jpkrohling @kovrus @mar4uk
• translator/loki: @gouthamve @jpkrohling @kovrus @mar4uk

See Adding Labels via Comments if you do not have permissions to add labels yourself.

[jpkrohling]

@domasx2, is this still relevant? I remember you mentioning that you might not even use logfmt after all.

[domasx2]

This is implemented: #15286 my bad, should have closed the issue