support routing by custom attribute for spans

[alburthoffman]

Component(s)

exporter/loadbalancing

Is your feature request related to a problem? Please describe.

At present it only supports routing keys like service, traceID for spans.
we meet an issue that down stream service has unbalanced traffic due to distribute by service.
some services have much higher traffic and some are idle.

if distributing by traceid, each downstream pod would hold all labels which requires lots of memory.

Describe the solution you'd like

the idea is to allow a custom attribute, so we can do something like service + operation.

Describe alternatives you've considered

no alternatives for now. we suffer from this.

Additional context

No response

[github-actions]

Pinging code owners:

• exporter/loadbalancing: @jpkrohling @rlankfo

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[rlankfo]

This PR was merged last month to support an "attributes" routing_key.

attributes: Routes based on values in the attributes of the traces. This is similar to service, but useful for situations in which a single service overwhelms any given instance of the collector, and should be split over multiple collectors.

Does this solve your use case?