Document how to configure alternative load balancers for OTel collectors

[jmacd]

The OTel collector output by the builder does not exactly allow the user to configure alternative gRPC load balancers, to do so would require a custom extension. The builder could easily be extended with a new section of _-import block to bring in Go modules that register gRPC load balancers, which would allow other (better) load balancers to be used.

The out-of-the-box OTel Collector supports only round_robin and pick_first.