RoundedDoubleConverterTest#testRoundingAndTextFormat fails for German locale

## Describe the bug
When the (system) locale is de_DE RoundedDoubleConverterTest#testRoundingAndTextFormat fails with:
```
org.opentest4j.AssertionFailedError: 

expected: "0.5556"
but was : "0,5556"
```

## Steps to reproduce
1. Set the system locale to de_DE
2. Run `mvn test -pl :apm-agent-core -Dtest=RoundedDoubleConverterTest`

## Expected behavior
All tests pass
