pub fn iter_mut(&mut self) -> IterMut<'_> {
        IterMut {
            inner: self.headers.iter_mut(),
        }
    }
An iterator visiting all key-value pairs, with mutable value references.

The iterator order is arbitrary, but consistent across platforms for the
same crate version. Each key will be yielded once per associated value,
so if a key has 3 associated values, it will be yielded 3 times.

# Examples

```
# use tonic::metadata::*;
let mut map = MetadataMap::new();

map.insert("x-word", "hello".parse().unwrap());
map.append("x-word", "goodbye".parse().unwrap());
map.insert("x-number", "123".parse().unwrap());

for key_and_value in map.iter_mut() {
match key_and_value {
KeyAndMutValueRef::Ascii(key, mut value) =>
value.set_sensitive(true),
KeyAndMutValueRef::Binary(key, mut value) =>
value.set_sensitive(false),
}
}
```
tonic/src/metadata/map.rs