fix(observability)!: Correct all `*_nanoseconds` metrics to `_seconds` (#7373)

* fix(observability): Correct all `*_nanoseconds` metrics to `_seconds`

All the referenced metrics labelled as "nanoseconds" are passed a
`Duration` as the value, which the `metrics` crate automatically
converts into (fractional) seconds internally. As such, all these names
are incorrect and need correction.

* Fix some more named `*_ns` but were really seconds too

Signed-off-by: Bruce Guenter <bruce.guenter@datadoghq.com>