[WIP] Add support for Server Timing headers

[stipsan]

This PR is not merge ready, I'm opening it early to discuss the prospect of the feature itself and if I'm on the right track with it or not.

The Server Timing headers enable more information in the Chrome DevTools on each individual request:

View larger image with more context.

Server Timing information is complimentary to existing tracing information. Some of its constraints is that you can only specify a duration per field, no start or end times to create a timeline context in relation to the other fields that might happen in parallel or before/after one another.

It's very suitable to show a brief overview over where the most time were spent in a query. This PR walks through the tracing information in the response and reduces it to the longest duration per path.

Looking forward to your feedback 😄

TODO:

• Update CHANGELOG.md with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

Pull Request Labels

• feature
• blocking
• docs

[pranaygp]

ETA on this being merged @abernix / @stipsan?

I'm considering rolling it myself (referencing the work done in this PR) for my apollo server instance

[stipsan]

@pranaygp well I’ve been waiting for feedback for a while now. I wouldn’t hold my breath 😉

[glasser]

I think this could be built as a plugin using willResolveField just like the usage reporting plugin does. (When this PR was written, willResolveField didn't exist; it was added in #3988.) It thus doesn't need to be part of the core project.