Return 0 from RateCounter.rate() when no values have been recorded

[vasiliy-mikhailov]

RateCounter.rate() computes sum / (double) countLength. Before any value is
recorded countLength is 0, so the method returns NaN. Callers reading the
rate of a fresh counter then propagate NaN (and NaN compares false against
every threshold).

Return 0.0 when nothing has been recorded.

Verifying this change

The added RateCounterTest fails on the current master and passes with this change:

Before the fix (on master):

$ mvn test -Dtest=RateCounterTest -pl core
[INFO] Running com.taobao.arthas.core.util.metrics.RateCounterTest
[ERROR] Tests run: 4, Failures: 2, Errors: 0, Skipped: 0 <<< FAILURE!
org.opentest4j.AssertionFailedError: RateCounter.rate() should not return NaN when no values have been updated, but got: NaN ==> expected: <false> but was: <true>
	at com.taobao.arthas.core.util.metrics.RateCounterTest.testRateWhenNoUpdatesShouldNotBeNaN(RateCounterTest.java:18)
[INFO] BUILD FAILURE

After the fix:

$ mvn test -Dtest=RateCounterTest -pl core
[INFO] Running com.taobao.arthas.core.util.metrics.RateCounterTest
[INFO] Tests run: 4, Failures: 0, Errors: 0, Skipped: 0
[INFO] BUILD SUCCESS

AI assistance disclosure

This contribution was produced with the help of an AI pipeline. The pipeline processed a large amount of source code to surface suspected bugs, reproduced a subset of them with failing unit tests and generated candidate fixes, and prepared pull requests from the ones that held up. Each PR was then reviewed and verified by a human before being opened: the fix and test were checked by hand and the test was confirmed to fail before the change and pass after.