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
* Sent resource calls for metadata to the backend
* moved resource calls to the backend
* code review feedback
* fixed post with body
* statuscode >= 300
* cleanup
* fixed tests
* fixed datasource tests
* code review feedback
* force some other endpoints to only GET
* fix linting errors
* fixed tests
* was able to remove section of redundant code
* cleanup and code review feedback
* moved query_exemplars to get request
* fixed return on error
* went back to resource calls, but using the backendsrv directly
* moved to a resource call with fallback
* fixed tests
* check for proper messages
* proper check for invalid calls
* code review changes
