feat: add 'service_target_*' fields to dropped spans metrics

[kruskall]

Part of the implementation plan for span service target.

See https://github.com/elastic/apm/blob/main/specs/agents/tracing-spans-service-target.md

Closes https://github.com/elastic/apm-agent-go/issues/1291

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• Start Time: 2022-09-14T11:04:36.851+0000

• Duration: 60 min 43 sec

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[simitt]

@kruskall @axw is this blocked by anything or is the only part missing to add a test?

[axw]

AFAICS this just needs a test.

[kruskall]

I've updated the test!

Should we remove destination service resource from dropped spans ?