Extracting all keys from CallOptions objects

In [gax-java](https://github.com/googleapis/gax-java/), we would like to be able to merge two [CallOptions](https://github.com/grpc/grpc-java/blob/master/core/src/main/java/io/grpc/CallOptions.java) objects together. We are able to partially implement this merging of `CallOptions` using the `with` methods available on `CallOptions`, such as here: https://github.com/googleapis/gax-java/blob/master/gax-grpc/src/main/java/com/google/api/gax/grpc/GrpcCallContext.java#L223

However, currently it is not possible to extract the full set of custom options from the `CallOptions` object, which means we are not able to create a merged set of custom options.

Is there any way to extract the full set of keys from a `CallOptions` object currently? If not, would grpc-java consider adding either (a) a `merge()` method on `CallOptions`, or (b) a way to extract the complete set of custom options, to allow us to implement merging?

Inizia con la classe CallOptions di grpc-java e il riferimento gax-java GrpcCallContext.java intorno alla riga 223. Determina se la modifica richiesta debba esporre chiavi di opzioni personalizzate o fornire un’unione, e definisci il comportamento dell’API e i test necessari per verificare che due oggetti CallOptions possano essere combinati.

Scritto dal modello di indicizzazione a partire dal testo della issue.