adapter: avoid more tracing ref panics in explain codepaths

[guswynn]

I cant find where these .await points were added (these were refactored and move multiple times recently, but any use of tokio sync objects can cause this issue

@aljoscha @aalexandrov do you know if these calls to sequence_peek_timestamp were originally added long enough ago that its in this weeks release? if so this needs to be backported

Motivation

• This PR fixes a recognized bug.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes:

[maddyblue]

These were made async in #22091 from October 2.

[guswynn]

@mjibson sounds good, we can just fix forward then!

[aljoscha]

I confirmed that this does indeed fixed the failures I saw in nightly runs of my oracle PR!