Capture and emit function name in log messages

[chenBright]

This PR adds the function name (__func__) to absl log messages, making it appear in the log prefix right after the line number, which gives a more precise callsite identifier.

# before
I0102 03:04:05.678900     451 foo.cc:1234] RAW: hello world

# after
I0102 03:04:05.678900     451 foo.cc:1234 MyFunc] RAW: hello world

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[chenBright]

I have signed the CLA.