Ticks APIs support time measurement in different units. However, their return types are inconsistent, `double` vs `uint64_t`.

For example, in `ElapsedCounterSource`
```
   static double seconds(Type value);
   static uint64_t milliseconds(Type value);
   static uint64_t microseconds(Type value);
   static uint64_t nanoseconds(Type value);
```

Such inconsistency causes some inconvenience to some callers expecting lossless measurements in milliseconds, e.g. many `* 1000.0` in `g1CollectedHeap.cpp`.

One can standardize the return type to `double` and ask the caller to cast integer types if needed.