Fix flaky GaugeTest.testTimer test

[jaydeluca]

From: https://github.com/prometheus/client_java/actions/runs/23299167470/job/67754952796?pr=1967

Error:  Tests run: 12, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 0.396 s <<< FAILURE! -- in io.prometheus.metrics.core.metrics.GaugeTest
Error:  io.prometheus.metrics.core.metrics.GaugeTest.testTimer -- Time elapsed: 0.034 s <<< FAILURE!
java.lang.AssertionError: 

Expecting actual:
  0.022711679
to be close to:
  0.012
by less than 0.005 but difference was 0.010711679.
(a difference of exactly 0.005 being considered valid)
	at io.prometheus.metrics.core.metrics.GaugeTest.testTimer(GaugeTest.java:91)