The Routing team is moving from cloudfoundry/lager to uber-go/zap logging
library for the Gorouter log, as we have observed improvements to
throughput performance using zap. This change has no impact on the
access.log, only to gorouter.log.

The Gorouter log message will change in the following ways. Please let us
know if any of these are a concern. Thank you!

1. "session" removed from "data" key. Does anyone use this?

lager
"data":{"host":"10.0.32.21:4222","session":"1"}

zap
"data":{"host":"10.0.32.21:4222"}


2. Values for "source" key more granular. Seems helpful for filtering
messages for like operations

lager
"source":"vcap.gorouter"

zap
"source":"vcap.gorouter"
"source":"vcap.gorouter.nats"
"source":"vcap.gorouter.subscriber"
"source":"vcap.gorouter.router"
"source":"vcap.gorouter.registry"


3. Source no longer prepends value of "message" key, as it is redundant
to the "source" key.

lager
"message":"vcap.gorouter.starting"

zap
"message":"starting"

4. Key reordering. My expectation is that no one should be using key order
for log analysis.

lager
{"timestamp","source","message","log_level","data"}

zap
{"log_level","timestamp","message","source","data"}


Full example log messages: https://gist.github.com/shalako/
79c8d7e8bf6d26b9934bb2679914c856

Thank you,

Shannon Coen
Product Manager, Cloud Foundry
Pivotal, Inc.