Duplicate Delayed LB pick complete event emitted by old pick_first policy.

[vinothkumarr227]

The old pick_first LB sends the "Delayed LB pick complete" event twice due to redundant picker updates—once on the first resolver update and again when the subchannel connects. The new pick_first correctly de-duplicates these events.

Reference Issues: #8342 (comment)

[dfawley]

This is a bug with how we implemented the stats callout; it was only intended to be emitted once per attempt, but we did it every time the LB policy asked for the attempt to be queued. This should be fixed by #8465.