Print trailing metadata in the "grpc_cli call" command.

[cookyt]

This often includes important debugging information when the RPC returns
a non-OK status.

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[y-zeng]

Can we only print the trailing metadata when the RPC returns a non-OK status?

[cookyt]

Trailing metadata could be useful even for OK responses, and the PrintMetadata function will only print metadata if the given metadata parameter is non-empty, so unconditionally printing it won't create unneeded terminal spam.

[grpc-testing]

****************************************************************

libgrpc.so

     VM SIZE        FILE SIZE
 ++++++++++++++  ++++++++++++++

  [ = ]       0        0  [ = ]


****************************************************************

libgrpc++.so

     VM SIZE        FILE SIZE
 ++++++++++++++  ++++++++++++++

  [ = ]       0        0  [ = ]

[grpc-testing]

[trickle] No significant performance differences

[grpc-testing]

[microbenchmarks] No significant performance differences