Fix potential metric loss when open_buffer is combined with disable_buffering=False

[vickenty]

What does this PR do?

Fix potential metric loss when open_buffer is combined with disable_buffering=False.

Description of the Change

Remove call to _reset_buffer from open_buffer.

When disable_buffering=False, the buffer may contain still unsent metrics, which are lost when we reset the buffer.

Alternate Designs

Possible Drawbacks

Verification Process

Additional Notes

Release Notes

Review checklist (to be filled by reviewers)

• [ ] Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• [ ] PR title must be written as a CHANGELOG entry (see why)
• [ ] Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• [ ] PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• [ ] PR should not have do-not-merge/ label attached.
• [ ] If Applicable, issue must have kind/ and severity/ labels attached at least.