runtime/gateway: add hostname, pid, zone fields to logger

[Raynos]

This PR adds some default fields to the logger, namely :

• hostname
• pid
• zone

This will improve basic logs to verify where this log came
from in terms of dc / hostname, etc.

r: @uber/zanzibar-team

[coveralls]

Coverage decreased (-0.005%) to 71.026% when pulling 19a68e5 on hostname-pid-zone into 2dd19f1 on log-fields.

[coveralls]

Coverage increased (+0.03%) to 73.816% when pulling 502e1dd on hostname-pid-zone into d8685e9 on master.