feat: add "total successful" metrics for signature and signer registrations

[Alenar]

Content

This PR add two new metrics to the aggregator:

• signer_registration_total_successful_since_startup
• signature_registration_total_successful_since_startup

Crossed with the existing "total received" metrics allow to track success or failure of signature and signer registrations using the aggregator Prometheus metrics.

Also:

• Fix SignatureProcessor only incrementing metric signature_registration_total_received_since_startup on signature registration success (which is covered by the new metric) instead of when it have been acknowledge by the system.

Pre-submit checklist

• Branch
  • Tests are provided (if possible)
  • Crates versions are updated (if relevant)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
• PR
  • All check jobs of the CI have succeeded
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested
• Documentation
  • No new TODOs introduced

Issue(s)

Closes #2855

[github-actions]

Test Results

    4 files  ±0    169 suites  ±0   23m 2s ⏱️ +30s
2 275 tests +4  2 275 ✅ +4  0 💤 ±0  0 ❌ ±0 
7 136 runs  +8  7 136 ✅ +8  0 💤 ±0  0 ❌ ±0 

Results for commit 3b3440c. ± Comparison against base commit ac2db70.

This pull request removes 2 and adds 6 tests. Note that renamed tests count towards both.

mithril-aggregator ‑ http_server::routes::signatures_routes::tests::test_register_signatures_increments_signature_registration_total_received_since_startup_metric
mithril-aggregator ‑ http_server::routes::signer_routes::tests::test_register_signer_post_increments_signer_registration_total_received_since_startup_metric

mithril-aggregator ‑ http_server::routes::signatures_routes::tests::test_register_signatures_increments_signature_registration_total_received_and_successful_since_startup_metric_if_successful
mithril-aggregator ‑ http_server::routes::signatures_routes::tests::test_register_signatures_only_increments_signature_registration_total_received_since_startup_metric_if_failure
mithril-aggregator ‑ http_server::routes::signer_routes::tests::test_register_signer_increments_signer_registration_total_received_and_successful_since_startup_metric_if_successful
mithril-aggregator ‑ http_server::routes::signer_routes::tests::test_register_signer_only_increments_signer_registration_total_received_since_startup_metric_if_failure
mithril-aggregator ‑ services::signature_processor::tests::processor_process_signatures_send_only_total_received_statistic_if_failure
mithril-aggregator ‑ services::signature_processor::tests::processor_process_signatures_send_total_received_and_successful_statistics_if_successful

♻️ This comment has been updated with latest results.

[jpraynaud]

LGTM 👍