Add Generic Log functions with level via argument

[lugray]

I wanted to be able to use Log with a variable level, and it seemed like I should add Logf and Logln if I was going to add Log.

[richpoirier]

Hit Approve too fast.

[richpoirier]

@lugray Do you only need Log() on Entry? I would think you'd want it on Logger?

Maybe some more context in the description would help -- some examples.

[lugray]

@lugray Do you only need Log() on Entry? I would think you'd want it on Logger?

In my case I did only need it on Entry, but you're right, it makes sense to make the change on Logger too, so I did that.

An example of why you'd want this, and where I use it:

const ignoreError logrus.Level = math.MaxInt32

func runAndLogGitCmd(ctx context.Context, cmd shell.Supervisor, defaultLevel logrus.Level, levelOverrides map[string]logrus.Level) (stdout []byte, err error) {
	stdout, stderr, err := cmd.RunAndGetOutput()
	if err == nil {
		return stdout, nil
	}

	gitErr := parseGitCmdError(string(stderr), err)

	level := defaultLevel
	if levelOverrides != nil {
		if l, ok := levelOverrides[gitErr.ID()]; ok {
			level = l
		}
	}

	if level == ignoreError {
		return stdout, nil
	}

	log(ctx, gitErr).WithFields(gitErr.LogFields()).Log(level, "error while running git command")
	return stdout, gitErr
}

Some git errors in our app should be fully ignored, others should be downgraded to WarnLevel, most should be ErrorLevel, but some commands are generally allowed to fail and so called with a defaultLevel of WarnLevel.

[richpoirier]

@lugray Maybe add a TestLog() test case to logrus_test.go and then we'll be good to merge.

[lugray]

Added test - thanks for the review on this.