Prometheus: Fix X-Id-Token and X-ID-Token sent to Prometheus in query requests (#60342)
* Prometheus: Use Set rather than map assignment in sdkHeaderToHttpHeader
Fixes#59940
* Prometheus: Add TestPrometheusCanonicalHeaders
(cherry picked from commit 92c01e416e)
Co-authored-by: Giuseppe Guerra <giuseppe.guerra@grafana.com>
* Introduce a new feature flag for prometheus buffered client
* Use querydata client as default and put buffered client behind the feature flag
* Remove prometheusStreamingJSONParser feature flag as it is not needed anymore
* Update tests
* Fix unit tests
* Update feature flag description