pub fn to_str(&self) -> Result<&str, ToStrError> {
        self.inner.to_str().map_err(|_| ToStrError::new())
    }
Yields a `&str` slice if the `MetadataValue` only contains visible ASCII
chars.

This function will perform a scan of the metadata value, checking all the
characters.

# Examples

```
# use tonic::metadata::*;
let val = AsciiMetadataValue::from_static("hello");
assert_eq!(val.to_str().unwrap(), "hello");
```
tonic/src/metadata/value.rs