added error_code to the metrics

Add err code to prometheus metrics (#2)

* Add err code to prometheus metrics

* Move const to a group

* Pull changes from new-gqlgen

* Codereview fixes

* Refactor

Co-authored-by: Alexey Ankip <aankip@humans.net>

spans only for object resolvers

Merge pull request #1 from humans-group/improve_tracing

improve tracer

panic fixed

v0.0.5

add nil check

v0.0.4

opCtx.Operation could be nil

small fixes

v0.0.3

opCtx.Operation could be nil

small fixes

v0.0.2

opCtx.Operation could be nil

initial fork

v0.0.1

refactored opentracing and prometheus only