Mixin: Allow specifying an instance name filter

This commit allow specifying the instance name filter, in order to
filter the datasources shown on the dashboards.

For example, when generating the dashboards one can do the following
(i.e in config.libsonnet)

```
  dashboard+:: {
    prefix: 'Thanos / ',
    ...
    instance_name_filter: '/EU.*/'
```

Signed-off-by: Jacob Baungard Hansen <jacobbaungard@redhat.com>

CHANGELOG.md

@@ -15,6 +15,7 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 - [#6185](https://github.com/thanos-io/thanos/pull/6185) Tracing: tracing in OTLP support configuring service_name.
 - [#6192](https://github.com/thanos-io/thanos/pull/6192) Store: add flag `bucket-web-label` to select the label to use as timeline title in web UI
 - [#6167](https://github.com/thanos-io/thanos/pull/6195) Receive: add flag `tsdb.too-far-in-future.time-window` to prevent clock skewed samples to pollute TSDB head and block all valid incoming samples.
+- [#6273](https://github.com/thanos-io/thanos/pull/6273) Mixin: Allow specifying an instance name filter in dashboards
 
 ### Fixed
 

mixin/dashboards/defaults.libsonnet

@@ -9,6 +9,7 @@ local utils = import '../lib/utils.libsonnet';
     prefix: 'Thanos / ',
     tags: error 'must provide dashboard tags',
     timezone: 'UTC',
+    instance_name_filter: '',
   },
 
   // Automatically add a uid to each dashboard based on the base64 encoding
@@ -37,7 +38,13 @@ local utils = import '../lib/utils.libsonnet';
       ],
 
       templating+: {
-        list+: [
+        // Add optional filter to the datasource template variable
+        list: [
+          if variable.name == 'datasource'
+          then variable { regex: thanos.dashboard.instance_name_filter }
+          else variable
+          for variable in super.list
+        ] + [
           template.interval(
             'interval',
             '5m,10m,30m,1h,6h,12h,auto',