fix(grpc): fix grpc span leak

[goodspark]

Upstreaming a fix by @samschlegel

Don't leak the spans when using the .future(..) interface

Checklist

• [x] Change(s) are motivated and described in the PR description.
• [x] Testing strategy is described if automated tests are not included in the PR.
• [ ] Risk is outlined (performance impact, potential for breakage, maintainability, etc).
• [ ] Change is maintainable (easy to change, telemetry, documentation).
• [x] Library release note guidelines are followed. If no release note is required, add label changelog/no-changelog.
• [ ] Documentation is included (in-code, generated user docs, public corp docs).
• [ ] Backport labels are set (if applicable)

Reviewer Checklist

• [ ] Title is accurate.
• [ ] No unnecessary changes are introduced.
• [ ] Description motivates each change.
• [ ] Avoids breaking API changes unless absolutely necessary.
• [ ] Testing strategy adequately addresses listed risk(s).
• [ ] Change is maintainable (easy to change, telemetry, documentation).
• [ ] Release note makes sense to a user of the library.
• [ ] Reviewer has explicitly acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment.
• [ ] Backport labels are set in a manner that is consistent with the release branch maintenance policy

[samschlegel]

Oh hey nice! I forgot I did this 😅

[github-actions[bot]]

This pull request has been automatically closed after a period of inactivity. After this much time, it will likely be easier to open a new pull request with the same changes than to update this one from the base branch. Please comment or reopen if you think this pull request was closed in error.