Calculate failed calls count

[bocharovf]

Requirement - what kind of business use case are you trying to solve?

Display amount of errors / error percent on Dependencies tab in Jaeger UI.

Problem - what in Jaeger blocks you from solving the requirement?

Currently we calculate only "callCount" - total amount of calls between two services whether successful or not.

Proposal - what do you suggest to solve the problem or improve the existing situation?

Calculate additional field "errorCount" - amount of calls between two services with error. Where error is determined from error tag equal to true.
It could be displayed in Jaeger UI "as is" or like "error percent".

Any open questions to address

[pavolloffay]

The functionality in this repository depends on the UI and storage data model.

To make this work we would have to change the UI and backend components to query and return these values.

This is somehow related to the generic issue "store tags in dependencies": jaegertracing/jaeger#1631