Pin gitlab-metrics-exporter version

It turns out we cannot use moving refs like branch
names in our release tooling. Setting this to a SHA
should allow us to re-add GME to our release pipeline
without failing deploys.

GITLAB_METRICS_EXPORTER_VERSION

-main
+2c781b65ccfd6d016c7bfab1982ecb234c4e2cd0

spec/lib/tasks/gitlab/metrics_exporter_task_spec.rb

@@ -26,7 +26,7 @@
     let(:expected_clone_params) do
       {
         repo: 'https://gitlab.com/gitlab-org/gitlab-metrics-exporter.git',
-        version: 'main',
+        version: an_instance_of(String),
         target_dir: 'path/to/exporter'
       }
     end