Describe the bug:

capture_span() types start as int instead of float

To Reproduce

1. apm_client.start_transaction()
2. t0 = time.time()
3. do_something()
4. t1 = time.time()
5. dur = t1-t0
6. do_something_else()
7. span = elasticapm.capture_span(start=int(t0), duration=dur)
8. with span as s: pass
9. apm_client.end_transaction()
10. Note that span will not fall in transaction time frame correctly.

Environment (please complete the following information)

• OS: mac
• Python version: 3.12.10
• Framework and version: FastApi 0.111.1
• APM Server version: 6.23.0
• Agent version: N/A

Additional context

elasticapm.capture_span() takes an optional argument start, which is typed to be an int. This works well enough with a time.time() float, but if you are using mypy, you will realize that it needs an int. Only problem is that capture_span() passes start to Span(BaseSpan), which calls super().__init__() which initializes start as a float, passing it through time_to_perf_counter(start). This method fails miserably with an int representation of time.time().