Add Unwrap methods to consumer errors

[bjsignalfx]

Description: Fixing a bug - Add Unwrap method to consumer errors permanent, Traces, Logs and Metrics. Functions Is and As in standard package errors use Unwrap to retrieve wrapped errors.

Testing: Created error type testErrorType. Wrapped testErrorType values with the consumer errors. Used function errors.As to retrieve wrapped testErrorType values. Added tests:

• TestPermanent_Unwrap
• TestTraces_Unwrap
• TestLogs_Unwrap
• TestMetrics_Unwrap

[tigrannajaryan]

Why do we need this? Is this necessary for some other code that is blocked by this?

[bjsignalfx]

Why do we need this? Is this necessary for some other code that is blocked by this?

I ran into a situation where consumererror.permanent is wrapped by consumererror.Logs and in that case IsPermanent() returns false because consumererror.Logs does not implement Unwrap. I also see that it may not make sense to wrap a permanent error with Logs, that is add failed logs to the permanent error, because it's not like you are going to retry sending. At the same time errors.As and errors.Is operations on consumer errors should work as expected.