Title: Log format: allow logging without source, or shortened source

Description:
Currently our logs look like [bazel-out/k8-opt/bin/external/envoy/source/common/config/_virtual_includes/grpc_stream_lib/common/config/grpc_stream.h:92] some log. Due to covid-19 WFH my monitor is really tiny so now the source part grpc_stream.h:92 takes up the entire screen.

It would be nice if there was some way to make this shorter. I looked over https://www.envoyproxy.io/docs/envoy/latest/operations/cli#cmdoption-log-format but did not see any, just %v which includes both the message and the source.

Just the tail part of the source might be a useful compromise as well (eg [grpc_stream.h:92] some log. If we wanted maximum flexibility, it seems like we could have

• %v full source + message
• %something full source
• %something trimmed source
• %something just the message

I am not sure that level is actually needed though