Specify naming scheme of default service name

[felixbarny]

Also turns some SHOULDs into MUSTs

• Create PR as draft
• Approval by at least one other agent
• Mark as Ready for Review (automatically requests reviews from all agents and PM via CODEOWNERS)
  • Remove PM from reviewers if impact on product is negligible
  • Remove agents from reviewers if the change is not relevant for them
• Merge after 2 business days passed without objections

Relates to https://github.com/elastic/observability-design/issues/21
/cc @formgeist

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• Start Time: 2021-11-29T16:35:44.227+0000

• Duration: 4 min 42 sec

• Commit: 8612e09

[trentm]

LGTM.

Just to say it out loud (and perhaps help searching), this will change the current unspec'd fallbacks of nodejs_service and python_service in the Node.js and Python agents.

[gregkalapos]

LGTM,

similarly to what @trentm says - in .NET the fallback is unknown and we'll need to change that if this gets merged. (good thing the fallback is really and edge case).

[vigneshshanmugam]

LGTM, we will switch to unknown in the RUM agent. Right now RUM agent does not have zero config support, I have opened elastic/apm-agent-rum-js#1103 to address this.

[felixbarny]

LGTM, we will switch to unknown in the RUM agent

For RUM, the default would be unknown-rum-js-service.