Style guide: Document avoidance of globals in Go code (#29803)

* Chore: Document avoidance of globals in Go code

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

contribute/style-guides/backend.md

@@ -39,3 +39,9 @@ Tests must use the standard library, `testing`. For assertions, prefer using [te
 The majority of our tests uses [GoConvey](http://goconvey.co/) but that's something we want to avoid going forward.
 
 In the `sqlstore` package we do database operations in tests and while some might say that's not suited for unit tests. We think they are fast enough and provide a lot of value.
+
+## Globals
+
+As a general rule of thumb, avoid using global variables, since they make the code difficult to maintain and reason
+about, and to write tests for. The Grafana codebase currently does use a lot of global variables, especially when
+it comes to configuration, but that is a problem we're trying to solve.