log: default StdlibAdapter file key should be "caller"

[willfaught]

Not "file". To match the naming of DefaultCaller/UTC.

Open to a PR?

[peterbourgon]

Hmm, on a first pass, I don't agree. File is considerably less ambiguous; caller could imply a lot of things. But I'm open to being persuaded. @ChrisHines, any opinion?

[willfaught]

Seems to me like the nomenclature should match since they both produce "file:line", whatever the term is. I'm fine with DefaultFile/UTC too.

[willfaught]

Although it's not just the file, it's also the line, so DefaultFile/UTC might be unclear; at least DefaultCaller/UTC implies as much info as file/line.

[ChrisHines]

I would like to note that only StdlibAdapter actually encodes the key string (and provides a default of "file"). The key of "caller" only appears in examples and tests. I don't think it is a big deal, but if forced to choose I would go with consistency and would agree with @willfaught.

[peterbourgon]

Oh, I see now. Yes, let's be consistent. I'm down with "caller".