export the _callerSkip to make AddCaller user can change the skip manually

[arthurkiller]

export the _callerSkip for user to change the skip manually

• [*] signed Uber's Contributor License Agreement;
• [*] added tests to cover your changes;
• [*] run the test suite locally (make test); and finally,
• [*] run the linters locally (make lint).

[mention-bot]

@arthurkiller, thanks for your PR! By analyzing the history of the files in this pull request, we identified @akshayjshah and @jcorbin to be potential reviewers.

[arthurkiller]

I don't understand why CI failed in go1.7 May be the CI trigger need to review

[arthurkiller]

On my Mac , make lint can perform well with go 1.7

[akshayjshah]

Hi @arthurkiller - for some context on this issue, see #40. You're right that the AddCaller hook isn't working well at the moment; however, the best fix isn't to export it and make callers configure it. Instead of being a global variable, it needs to be a per-logger field - a sampled logger and a plain logger need to skip different numbers of fields, for example.

@jcorbin is working on this in his dev branch.