otel: plan out new signals and their attributes

[deniseli]

This needs to include:

From Alec's original issue desc: PubSub - count per topic, subscriber cursor, etc.

From Brad: counts for all the event types, queue depth (useful for most or all of the queues, probably)

Broadly speaking, the task here is to create a list of metrics + the attributes to include for each metric. We want as few metrics as possible, with the bulk of our analysis being performed by segmenting signals by attribute.

[deniseli]

Gonna go ahead and mark this as done since we are finally at the finish line on all the core metrics :D