Add convenience accessor functions on metadata

[robsignorelli]

When you have more than one value to grab from metadata, it can be fairly inconvenient to do this:

var someString string
var someInt int
metadata.Value(ctx, "foo", &someString)
metadata.Value(ctx, "bar", &someInt)

It would be nice if we could do something like this which feels more natural.

someString := metadata.Stringctx, "foo")
someInt := metadata.Int(ctx, "bar")

When generics land, maybe we can even do something like:

someStruct := metadata.Value(ctx, "bar", &MyStructType{})

But for now convenience functions for basic types will go a long way.