Refactor specs to avoid expecting implementation

[Bodacious]

What

Refactor the test suite to avoid writing tests that rely on specific implementation details (e.g. method calls).

Why

We want the test suite for these objects to be as robust as possible. The previous implementation was testing that particualr methods were called, rather than testing expected outcomes.

How

Adds RSpec matchers to make better assertions on Prometheus metrics.

Refactor to use new metrics.

Use RSpec built-in helpers where appropriate.

[Bodacious]

Moved back to draft. I think these changes are too much for one PR, and will split it into smaller parts.