Investigate bulk metadata functions

[carl-mastrangelo]

Metadata provides limited public APIs for interaction, typically requiring a Metadata.Key to do any reads or writes. This is useful to an application, which may know the key name and marshaller ahead of time, but not so much for middle-ware which may need to scan for keys. As an example, a middle-ware that looks for metadata prefixes and builds a custom Key depending on the suffix of the key name has trouble with the existing Metadata API. It requires multiple iterations through the metadata, and up to O(n^2) ops when trying to extract values with duplicate key names.

Metadata is also mutable, and does not want to expose internals, as they may yet change. I am proposing some experimental APIs here to address the bulk metadata functions use case:

Option 1:

// For each value in the MD which matches the key prefix, return the parsed value  
<T> List<T> walk(String keyPrefix, Metadata.BinaryMarshaller<T> marshaller)  {}

Option 2:

<T> void walk(Function<? super String, Optional<Metadata.Key<T>> nameToKeyFn, Consumer<? super Object> valueConsumer)  {}

Option 3:

// For each value in the MD which matches the key prefix, return the parsed value  
void walk(String keyPrefix, Metadata.BinaryMarshaller<?> marshaller, BiConsumer<? super String, ? super Object)  {}

Option 4:

void walk(Function<? super String, Optional<Metadata.BinaryMarshaller<?>> nameToKeyFn, BiConsumer<? super String, ? super Object)  {}

The different options provide the equivalent of a map operation (we might also be able to provide the equivalent of a Java8 stream). Option 1 is the least flexible, but has the simplest API. Option 2 allows the caller to provide their own Key producer based on the key name, but the value consumer doesnt get to know the key name used. Option 3 still uses the same marshaller for each key, but allows the consumer to know the key name. Option 4 is a combination of 2 and 3, allowing both dynamic marshaller construction based on the key name, and allowing the consumer of the the produced values to know the keyname (perhaps for later caching?)

[carl-mastrangelo]

cc @tsalomie @ejona86