Add `spacetime_worker_reducer_returned_errors_total` metric

[drogus]

Description of Changes

Currently, we have a metric for reducer panics called spacetime_worker_wasm_instance_errors_total. This commit adds a metric for tracking errors returned from the module, like for example an Err result in Rust, or throwing a SenderError in TypeScript

This will be needed for the web database overview dashboard, but it should be useful for tracking sender errors in general.

Expected complexity level and risk

1

Testing

• [x] I've tested the change locally verifying the counter increments when an instance returns an error

[drogus]

Just think we should change/simplify the naming to not reference wasm

Yeah, that makes sense. I wanted to go with the current convention, but we'll eventually probably rename it, too