Make Tower.Event support a service name

[msutkowski]

This is a small proposal that would ultimately build on the use case mentioned in #118. Currently, Tower always assumes that the error it is processing is from the current application. While this is going to be correct the majority of the time, we could make this slightly more flexible by introducing a service key to the struct that defaults to the current :otp_app (while allowing for an override). The idea here is that this is pretty minimal effort to support, but opens the door for someone to configure Tower as a centralized hub for errors from N services.

The other option is we could find a way to leverage the existing metadata for this purpose and just not consider this at all, but I believe we'd have to deal with this comment:

# Not beautiful that tower needs to know about something specific to tower_error_tracker.
# We may refactor this to be configurable by reporters or it can change once we support
# something like `Tower.set_context`.

Some possible options for making this work to start a discussion:

1. In application config -- config :tower, service: :my_great_service
2. Tower.set_context support
3. Extend logger_metadata env behavior (I don't really see this one working out at all, but it's possible)

Just to be clear, the goal is ultimately to find the best mechanism to say "I am an error being reported by app/service X". It's possible that this might just be a terrible idea, and that's okay! This is all still possible in user land code, but requires a bit of effort to achieve the same thing.

If this idea moves forward, I can open a similar discussion in ErrorTracker for how we make slight changes to support service aggregation.

Thanks for your time!

References:

• https://docs.datadoghq.com/getting_started/tagging/#overview (i don't think we'd need to support tags like this, but you can see how service is used)