Use epoch milli to check span start/stop

[ttddyy]

Prior to this change, TracerAssert#onlySpan randomly failed.

This was because it had followings to check a span has started/stopped.

assertTrue(span.getStartTimestamp().getNano() > 0, "Span must be started");
assertTrue(span.getEndTimestamp().getNano() > 0, "Span must be finished");

The getNano() could return 0 for valid Instant.

For example:

Instant i = Instant.parse("2022-09-07T00:33:58Z");
assertThat(i.getNano()).isEqualTo(0);

i = Instant.ofEpochMilli(1662510838000L);
assertThat(i.getNano()).isEqualTo(0);

Instead of using getNano(), this PR changes to constantly use toEpochMilli() > 0 to check whether the span has started or stopped.

Instant.ofEpochMilli(0).toEpochMilli() returns 0 whereas Instant.ofEpochMilli(10).toEpochMilli() returns 10.