Clock (OpenTelemetry SDK Common)

```
@ThreadSafe
public interface Clock
```
Interface for getting the current time.

Method Summary

All Methods Static Methods Instance Methods Abstract Methods
Modifier and Type	Method	Description
`static Clock`	`getDefault()`	Returns a default `Clock` which reads from system time.
`long`	`nanoTime()`	Returns a time measurement with nanosecond precision that can only be used to calculate elapsed time.
`long`	`now()`	Returns the current epoch timestamp in nanos from this clock.

- Method Detail
  - getDefault
```
static Clock getDefault()
```
    Returns a default Clock which reads from system time.
  - now
```
long now()
```
    Returns the current epoch timestamp in nanos from this clock. This timestamp should only be used to compute a current time. To compute a duration, timestamps should always be obtained using nanoTime(). For example, this usage is correct.
    long startNanos = clock.nanoTime(); // Spend time... long durationNanos = clock.nanoTime() - startNanos;
    
    This usage is NOT correct.
    long startNanos = clock.now(); // Spend time... long durationNanos = clock.now() - startNanos;
  - nanoTime
```
long nanoTime()
```
    Returns a time measurement with nanosecond precision that can only be used to calculate elapsed time.