fix(metrics): speckle_server_apollo_calls built with correct register

[iainsproat]

Description & motivation

speckle_server_apollo_calls was no longer present in the Prometheus metrics. It is not known when that metric stopped being produced.

This PR fixes the metric so that it is now available when Prometheus is scraped.

Changes:

• the metric is initialized when the plugin is initialized, not when the module is loaded
• the metric is initialized with the Prometheus register being explicitly provided
  • (we believe the bug that is being resolved is due to the metric being registered in a different Prometheus register than the one being served at /metrics)

To-do before merge:

Screenshots:

Validation of changes:

Metric speckle_server_apollo_calls is now present when /metrics endpoint is queried:

Checklist:

• My pull request follows the guidelines in the Contributing guide?
• My pull request does not duplicate any other open Pull Requests for the same update/change?
• My commits are related to the pull request and do not amend unrelated code or documentation.
• My code follows a similar style to existing code.
• I have added appropriate tests.
• I have updated or added relevant documentation.

References

[linear]

CXPLA-152 Investigate the undefined subscription connection (and label it)