otel: support sending to two distinct destinations

[deniseli]

THIS IS LOOOOWWWW PRIORITY! (because it isn't relevant until we move onto a big customer down the road, and even then it's a maybe. We DO NOT need this for the upcoming launch.)

This will be needed if we have a customer who isn't just running FTL in the same cluster as all their client code. We'll need to make sure the global otel setup we do in FTL is opt-in-able (or possibly opt-out-able?) by the customer team so that it consistently applies to both destinations.

[wesbillman]

Remove next label since this is low priority