Dd-trace-rb: Support for custom attributes on traces

[KieranP]

NewRelic tracer has the ability to send misc attributes along with the trace, particularly for knowing which user triggered the trace.

::NewRelic::Agent.add_custom_attributes({
  user_id: current_user.try(:id),
  user_name: current_user.try(:name),
})

Would be handy to be able to do something similar with Datadog so we can contact users affected in certain cases.

[delner]

Thus far we do allow for tagging all traces with particular attributes:

Datadog.configure do |c|
  c.tracer tags: { 'env' => 'prod' }
end

Alternativately, in your own code, you can access the current span and add tags as you see fit. e.g. in a Rails controller:

before_action :set_tags

def index
  # Some code...
end

private

def set_tags
  tracer = Datadog.configuration[:rails][:tracer]
  span = tracer.active_span
  span.set_tag('user_id' current_user.try(:id))
  span.set_tag('user_name' current_user.try(:name))
end

[KieranP]

@delner Perfect. Thank you. Would be good to have this documented on the Datadog Rails installation page for future reference for others.

[delner]

No problem. We did just add a reference to active_span to our documentation in 0.12-dev under "Manual Instrumentation" (although it isn't tagged as "custom attributes".)

Going to close this one since it seems like that'll work for you.