* prepare backend for structured metadata
* add `lokiStructuredMetadata` feature toggle
* use `lokiStructuredMetadata` feature flag
* add field type check to `labelTypesField`
* remove fixme
* fix feature toggle
* add field in dataplane mode
* use `data.Labels` where possible
* adjust framing tests
* improve verbiage
* improve naming
* update tests to "attributes"
Automatically forward core plugin request HTTP headers in outgoing HTTP requests.
Core datasource plugin authors don't have to specifically handle forwarding of HTTP
headers, e.g. do not have to "hardcode" the header-names in the datasource plugin,
if not having custom needs.
Fixes#57065
