Add Swift (SwiftPrometheus) to client libraries

[MrLotU]

This PR will add SwiftPrometheus (https://github.com/MrLotU/SwiftPrometheus) to the list of client libraries.

SwiftPrometheus is the current standard for Prometheus Client libraries in the ServerSide Swift ecosystem and is recognised as such by the SSWG. See SSWG Projects.

The library contains an example executable that can be used and altered to see what the output will look like.

Any questions and/or remarks are welcome.