Remove min value for `exit_span_min_duration`

[felixbarny]

Remove min value for exit_span_min_duration to allow opting out of dropping fast exit spans.

This may impact the existing implementations for .NET, Go, PHP, Python, and the in-progress implementation for Java. However, it seems most have not implemented a validation that would disallow users to set exit_span_min_duration to 0.

• [x] Create PR as draft
• [x] Approval by at least one other agent
• [x] 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
• [x] Merge after 2 business days passed without objections \ To auto-merge the PR, add /schedule YYYY-MM-DD to the PR description.

/schedule 2022-03-10

[apmmachine]

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#### Build stats * Start Time: 2022-03-10T10:01:36.749+0000 * Duration: 3 min 22 sec

[felixbarny]

This is scheduled to be merged in 2 days