Limithandler: change type label

What

Change the type label to limit_type for the following metrics:

• gitaly_pack_objects_acquiring_seconds
• gitaly_pack_objects_in_progress
• gitaly_pack_objects_queue
• gitaly_pack_objects_dropped_total

Why

For GitLab.com the type label is a reserved label that is used to specify the name of the service. Using the same label is causing collision and some render properly.

This also ends up using the same label as we do for logs which brings consistency.

I'm not sure if this will be considered a breaking change since we are changing the label of a metric that is hidden behind the feature flags pack_objects_limit_*

Reference: #5099 (closed)