Query planner metrics

[BrynCooke]

Add a selector to allow displaying metrics about query plans. Metrics should be accessible via context so that we can write a selector to obtain

• depth
• breadth
• anything else that is useful

[smyrick]

This would be really helpful for another large customer. We are tuning with APOLLO_FEDERATION_DEBUG=true when there is lots of use with @shareable it would also be maybe helpful to turn on a per-request debug mode that hade even more info for a given operation about the planning process, like:

• Number of paths computed
• Number steps used
• Number of edges checked
• Number of subgraphs considered

Also the outputted operation plan for each subgraph is a little different than the client operation, so other helpful stuff includes:

• Size of the subgraph operation in fields and bytes
• Aliases or requires overrides used