Fix clippy::needless_lifetimes warning

```
error: the following explicit lifetimes could be elided: 'a
  --> examples/with-metadata.rs:52:6
   |
52 | impl<'a, F: Future> Future for MeasureRuntime<'a, F> {
   |      ^^                                       ^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
   = note: `-D clippy::needless-lifetimes` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::needless_lifetimes)]`
help: elide the lifetimes
   |
52 - impl<'a, F: Future> Future for MeasureRuntime<'a, F> {
52 + impl<F: Future> Future for MeasureRuntime<'_, F> {
   |
```

examples/with-metadata.rs

@@ -49,7 +49,7 @@ pin_project! {
     }
 }
 
-impl<'a, F: Future> Future for MeasureRuntime<'a, F> {
+impl<F: Future> Future for MeasureRuntime<'_, F> {
     type Output = F::Output;
 
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {