[connector/spanmetrics] Fix flaky spanmetricsconnector test that relies on timing

[swar8080]

Description:
Tries fixing some flaky unit tests that relied on time.Now() always returning a higher value than the previous call

The solution is reading the current time from a clock.Clock interface. The test relying on ever-increasing Now() values uses a mock that always returns an increasing value.

Link to tracking Issue: #33438

Testing:

• Ran the component locally to verify the ticker still flushes after the configured duration passes

Closes #33438

[jcreixell]

LGTM

[crobert-1]

Failure in connector/servicegraph tests appears to be unrelated to this change, I've filed #33679 to address it.