DataLoaderInstrumentationContext (java-dataloader 5.0.0 API)

```
@PublicSpi
public interface DataLoaderInstrumentationContext<T>
```
When a DataLoaderInstrumentation.'beginXXX()' method is called then it must return a DataLoaderInstrumentationContext that will be invoked when the step is first dispatched and then when it completes. Sometimes this is effectively the same time whereas at other times it's when an asynchronous CompletableFuture completes.
This pattern of construction of an object then call back is intended to allow "timers" to be created that can instrument what has just happened or "loggers" to be called to record what has happened.

Method Summary

All Methods Instance Methods Default Methods
Modifier and Type	Method	Description
`default void`	`onCompleted(T result, java.lang.Throwable t)`	This is invoked when the instrumentation step is fully completed.
`default void`	`onDispatched()`	This is invoked when the instrumentation step is initially dispatched.

- Method Detail
  - onDispatched
```
default void onDispatched()
```
    This is invoked when the instrumentation step is initially dispatched. Note this is NOT the same time as the DataLoaderInstrumentation`beginXXX()` starts, but rather after all the inner work has been done.
  - onCompleted
```
default void onCompleted(T result,
                         java.lang.Throwable t)
```
    This is invoked when the instrumentation step is fully completed.
    
    Parameters:
    
    result - the result of the step (which may be null)
    
    t - this exception will be non-null if an exception was thrown during the step