search.scroll_time_in_millis might not be accurate

[jasontedor]

Metrics for scrolls (open scroll contexts, time scroll contexts held open, and completed scroll contexts) were added to the search stats in #9109. The time scroll contexts were held open is calculated at millisecond resolution using the time the context was created on the coordinating node. This can lead to two issues:

1. Millisecond resolution is too coarse to accurately capture the time that very short-lived scroll contexts are held open, especially on systems with low-granualarity clocks.
2. Discrepancies in the clocks between the coordinating node and the nodes where the scrolls are executed will lead to inaccuracies in measuring the time that the context is held open at the shard level.

Instead, an origin time in nanosecond resolution that is calculated on the executing node should be used to measure the time that scroll contexts are held open.