feat(mu): add metric to track size of trace database

[jfrain99]

Background

Currently, we have a database called trace.sqlite which holds a record of all logs in the MU for debugging purposes. There is a script, running on a cron hourly, which first checks the size of the database, and then deletes some old rows if it is too big. A metric which returns the size of the trace database will be helpful for ensuring that script is running and the database is not ballooning.

Solution

A new metric gauge, called ao_mu_trace_db_size, can be implemented. To determine the size of the sqlite database, we can leverage PRAGMA:

db.pragma('page_size', { simple: true })
db.pragma('page_count', { simple: true })

Then, we can multiple page_size (usually 4096 bytes) by page_count to find the size of the database. We can do this on metric collection.