Fix saturation point for shared runners

Fix saturation point for shared runners

Before this, we were measuring shared-runner utilization as follows:

gitlab_runner_jobs
/
gitlab_runner_concurrent

While the private and shared-gitlab-org shards we were doing:

gitlab_runner_jobs
/
gitlab_runner_limit

Because the labelset for gitlab_runner_concurrent does not include a system_id label, we were not recording for gitlab:component_saturation:ratio for the shared shard.

With this, we're using the same metric for the denominator for all shards (gitlab_runner_limit).

We're also making sure that we're including all shards into she shared_runners saturation component by only excluding private and shared-gitlab-org shards from that component. This means we'll start including the saas-linux shards and the macOS and windows runners. Because of this, we needed to add the shard label as a significant label, so we can distinguish which shard we're predicting saturation for in dashboards.

This has all been merged into a single file to keep the saturation point definitions the same for all of these shards. The only thing that changed for private_runners and shared_runners_gitlab is the wording in the description.