Query: Fix concurrency handling for mixed datasource queries (#70100)

* split queries and merge responses

* increase concurrency again

* update unit test to verify the headers are merged

* fix lint issue

* fix race condition in unit test

* Fix function name and add a bit more documentation about how the func should be used

* update function call after rename

* check for duplicate header vals

* make concurrent query limit configurable

* Update conf/sample.ini

Co-authored-by: Sofia Papagiannaki <1632407+papagian@users.noreply.github.com>

---------

Co-authored-by: Sofia Papagiannaki <1632407+papagian@users.noreply.github.com>

docs/sources/setup-grafana/configure-grafana/_index.md

@@ -1669,6 +1669,14 @@ Enable or disable the Profile section. Default is `enabled`.
 
 Enables the news feed section. Default is `true`
 
+<hr>
+
+## [query]
+
+### concurrent_query_limit
+
+Set the number of queries that can be executed concurrently in a mixed data source panel. Default is the number of CPUs.
+
 ## [query_history]
 
 Configures Query history in Explore.
@@ -1677,6 +1685,8 @@ Configures Query history in Explore.
 
 Enable or disable the Query history. Default is `enabled`.
 
+<hr>
+
 ## [metrics]
 
 For detailed instructions, refer to [Internal Grafana metrics]({{< relref "../set-up-grafana-monitoring" >}}).