Update timing strategy

[plantfansam]

A comment in opentelemetry-ruby reads:

      # Times are hard. Whenever an explicit timestamp is provided
      # (for Events or for the Span start_timestamp or end_timestamp),
      # we use that as the recorded timestamp. An implicit Event timestamp
      # and end_timestamp is computed as a monotonic clock offset from
      # the realtime start_timestamp. The realtime start_timestamp is
      # computed as a monotonic clock offset from the realtime
      # start_timestamp of its parent span, if available, or it is
      # fetched from the realtime system clock.
      #
      # We therefore have 3 start timestamps. The first two are used
      # internally (and by child spans) to compute other timestamps.
      # The last is the start timestamp actually recorded in the
      # SpanData.

I think we should update opentelemetry-lua to mirror the strategy in opentelemetry-ruby.