add AddError method to accumulator

[phemmer]

This adds an AddError() method to the accumulator. Ref #1446

As opposed to the original POC in #1446, I instead just implemented it so that AddError() immediately writes out to the log instead of sending it into a chan just so that a goroutine could then write it out to the log. This approach is much simpler.

I also added an errCount to the accumulator which will increment each time an error is generated. This was done for 2 reasons:

1. So that the Test() function could tell if any errors were generated during the run.
2. To pave the way for Telegraf should send stats about itself #1348.
   It would be very useful to know when a plugin starts throwing errors. You could then write a kapacitor alert to trigger when such starts happening.

Haven't updated CHANGELOG since the code in this PR is unused, and thus there is no user visible change.

[sparrc]

reading ac.inputConfig.Name could produce a race condition here. Since this bit of code shouldn't be hugely high-traffic, I would remove the atomic package, add a sync.Mutex to the accumulator, and change to:

    if err == nil {
        return
    }
    ac.Lock()
    ac.errCount++
    log.Printf("ERROR in input [%s]: %s", ac.inputConfig.Name, err)
    ac.Unlock()

[phemmer]

The only place where inputConfig.Name is touched is in internal/config/config.go:798 (during initialization). It is never read or written to ever again (prior to this PR it was an unused variable). Thus there's no need to protect it.

[sparrc]

yes but this function is reading it, so unless you make the function a value receiver, there can be a race condition here. nevermind, not sure why I was under the impression that reading struct fields was not thread-safe, should be fine...

[phemmer]

How do you want to handle all the failures due to test mocks not having the AddError() method (not satisfying the interface)?
Comment the line out of the interface until the next PR which starts converting the plugins? or do the conversion in this PR?

[sparrc]

You shouldn't have to update a bunch of test mocks, just add the AddError function to the testutil struct: https://github.com/influxdata/telegraf/blob/master/testutil/accumulator.go#L27

[phemmer]

Ah, didn't look into the issue too deeply (at all really). Will do that then :-)

[phemmer]

Updated PR with the testutil changes.

[sparrc]

looks great, thanks @phemmer