Add prometheus metrics to vacuum engine

[jgpruitt]

Description

Exposes three new prometheus metrics in the vacuum engine so we can more easily observe its behavior

• adds a gauge for the number of tables left to vacuum
• adds a counter for the number of tables vacuumed
• adds a gauge that roughly tracks the number of database connections currently in use by the vacuum engine

Since we limit our query to 1000 results, the tables left to vacuum gauge will never go above 1000 even if there are many more tables to be vacuumed.

1754

Merge requirements

Please take into account the following non-code changes that you may need to make with your PR:

• [ ] CHANGELOG entry for user-facing changes
• [ ] Updated the relevant documentation