feat: added error unwrapping

[FUSAKLA]

resolves #83

Hi, I discovered that when using error wrapping, the reported grpc_status is reported as an Unknown status. I encountered this issue in the Thanos project where for example context canceled error is normally reported as Cancelled gRPC status but once wrapped becomes Unknown.

The issue should be eventually solved in the go-grpc status package. There is even an issue for this grpc/grpc-go#2934. The problem is there are currently cca 3 ways to wrap errors and every one doing it different way.

I created a draft with bit hacky unwrapping of two of those, native Go unwrapping (commented out since added in 1.13.0 and I'm not sure how to deal with the dependency) and pkg/errors wrapf.

Not sure what is the right way to handle this, but currently this is quite inconvenient.
I'd be glad for any opinions on this.

Thanks!

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[FUSAKLA]

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[codecov-io]

Codecov Report

Merging #84 into master will increase coverage by 0.54%.
The diff coverage is 84.21%.

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   71.17%   71.71%   +0.54%     
==========================================
  Files           8        8              
  Lines         333      350      +17     
==========================================
+ Hits          237      251      +14     
- Misses         85       87       +2     
- Partials       11       12       +1

Impacted Files	Coverage Δ
server_metrics.go	80.5% <84.21%> (+0.31%)	⬆️

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[FUSAKLA]

@brancz PTAL if this is something you had in your mind?

[brancz]

Nice! lgtm 👍

[FUSAKLA]

Thanks!