fix: Fix tracing with graphql-ruby interpreter

[jturkel]

Attempting to use ApolloFederation::Tracing with the graphql-ruby interpreter runtime resulted in exceptions like this:

KeyError: key not found: :context
# ./lib/apollo-federation/tracing/tracer.rb:106:in `fetch'
# ./lib/apollo-federation/tracing/tracer.rb:106:in `execute_field'
# ./lib/apollo-federation/tracing/tracer.rb:50:in `trace'
# ...

This PR fixes that issue and resolves another issue with the field_type being set incorrectly in metrics recorded using the interpreter. The spec/apollo-federation/tracing_spec.rb diff looks kind of scary but I just moved all of the testing into a shared example group that could be reused with the legacy runtime and the interpreter runtime.

[srgoldman]

@noaelad any chance of getting this into an official release soon?

[jturkel]

Looks like this ended up being fixed in #59.