[DocDB] pg_txn_start_us unset for single shard transactions

[basavaraj29]

Jira Link: DB-11422

Description

Seems like pg_txn_start_us is not set for single shard transactions.

AsyncRpcBase<Req, Resp>::AsyncRpcBase(...)
  ...
  if (!metadata.transaction.transaction_id.IsNil()) {
    SetMetadata(metadata, data.need_metadata, &req_);

If so, this could lead to fairness issues among single shard vs distributed waiters since the waiter resumption logic would then depend only on the serial number of the waiting request which changes upon waiter re-entries.

Issue Type

kind/bug

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