furnace: Instrument makepkg builder

[onprem]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

• Instrument the Furnace package builder. The exposed metrics are
  • foundry_furnace_builder_builds_started_total: Total number of attempted builds by the builder.
  • foundry_furnace_builder_builds_completed_total: Total number of builds completed by the builder irrespective of success or failure.
  • foundry_furnace_builder_build_failures_total: Total number of builds attempted by the builder that failed.
  • foundry_furnace_builder_build_duration_seconds: Duration of builds completed successfully by the builder. This is a histogram.

Fixes #14

Verification

Added unit test for instrumentedBuilder.

[coveralls]

coverage: 62.733% (+11.9%) from 50.82%
when pulling 71c3ca4 on feat/metrics
into 21a5371 on main.