Rename fast field precision parameter to fast_precision

[dojiong]

The precision of span_start_timestamp_nanos and span_end_timestamp_nanos should be nanoseconds.

Edit from @guilload:
The conclusion of this conversation was to rename the fast field precision parameter to fast_precision. We can do so in a backward-compatible manner. The upcoming PR should also update the docs and the configuration examples in config/.

[guilload]

That's the fast field precision. The stored value remains in nanoseconds. The fast field won't compress as well with a precision in nanoseconds, that's why we opted for milliseconds since we were not sure it made sense for filtering and aggregations.

Do you have a use case for fast fields with nanoseconds precision?

@fmassot, @fulmicoton, do we want to revisit this decision?

[dojiong]

I'am exporing the tantivy index from quickwit's opentelementry traces index. I found the datetime field's precision is wrong:

1. the span_start_timestamp_nanos field in quickwit_opentelemetry::otlp::traces::Span use nanoseconds
2. the index's field use milliseconds
3. the field's data stored in tantivy index use microseconds

I think the precision of span_start_timestamp_nanos should be nanoseconds according to the field's name.

Or microseconds? (the data stored in index is in microseconds)

[dojiong]

Or rename precision to fast_precision indicating that is for fastfield?

[guilload]

3. the field's data stored in tantivy index use microseconds

We should be on a tantivy version that uses nanoseconds for storing datetimes (quickwit-oss/tantivy#2016)

Or rename precision to fast_precision indicating that is for fastfield?

I totally agree.

[guilload]

In the future, I want the OTel indexes to be "templatable" to let users customize their settings.