Instrumentation, api cleanup

[lpuskas]

• [X] error handling (contextual errors)
• [X] API - problem implementation (https://tools.ietf.org/html/rfc7807)
• [X] context propagation
• [x] instrumentation - metrics
• [x] instrumentation - tracing

[lpuskas]

closing as most of the issues have been sorted or partially done; refinements to be done in other issues