SDK should support an error callback

[dmathieu]

As mentioned in #162 (comment)

[jmacd]

The SDK itself also should support an OnError() callback so that it can issue errors to a handler configured by the application main() function. Probably every component (SDK, processor, exporter) should support this in a uniform way.

[akvanhar]

If this is something we expect exporters to handle in a uniform way, does it make sense to have it implemented here instead? A default OnError() call back could do log.Printf("Error when uploading spans to Exporter: %v", err). Even better if we know the name of the exporter in question.

[jmacd]

Discussed in the weekly SIG meeting. We support the idea of an interface with an OnError method that is passed to the constructors of each SDK, processor, exporter, and plugin that might need to report errors.

[freeformz]

@jmacd Do you mean an optional interface, such that the sdk checks to see if an exporter or the like is also an OnErrorer or the like and if so then uses this? Or something else?

[jmacd]

To me there are two questions here.
Should each connected group of SDK/processor/exporter/plugin share a single OnError handler? (If yes, how is this organized?)
Should there be a global OnError handler? (If no, will there be conditions where there's not a well defined OnError handler?)
One approach would be to have the SDK implement an OnError handler, and let the pre-initialization global SDK have a sensible default. This is slightly complicated because the exporter and processors are going to be constructed before the SDK. Should we somehow make the OnError handler a (standard, functional) option passed to all components?

Another trouble with this idea is that we haven't specified a type for the "SDK" itself. At present all we have are the idea of a Tracer and Meter factory (for named tracers and meters) plus the standing OTEP 0005 about global initialization: https://github.com/open-telemetry/oteps/pull/45/files

[jmacd]

I suggest this issue be named "SDK should to support an error callback". The current default metrics SDK supports one, but it should be supported by the tracer SDK as well.

[MrAlias]

Possibly worth looking into adding a global error handler (to be useful prior to sdk registration).