fix-disabling-telemetry-fc171a89

Fix merging of private values (i.e. meta)
Now allows for a meta key/value to be specified multiple times, with
the last value being the final value. In the example below, foo: "b"
will override foo: "a":

meta foo: "a"
meta foo: "b"

This allows disabling telemetry for a specific field that specifies a
resolver function, i.e.:

resolve &Foo.bar/3
meta absinthe_telemetry: false