pub fn with_capacity(capacity: usize) -> MetadataMap {
        MetadataMap {
            headers: http::HeaderMap::with_capacity(capacity),
        }
    }
Create an empty `MetadataMap` with the specified capacity.

The returned map will allocate internal storage in order to hold about
`capacity` elements without reallocating. However, this is a "best
effort" as there are usage patterns that could cause additional
allocations before `capacity` metadata entries are stored in the map.

More capacity than requested may be allocated.

# Examples

```
# use tonic::metadata::*;
let map: MetadataMap = MetadataMap::with_capacity(10);

assert!(map.is_empty());
assert!(map.capacity() >= 10);
```
tonic/src/metadata/map.rs