Which problem is this PR solving?

At the moment, GraphQL "resolve" spans (spans generated for each resolver/sub-resolver in a GraphQL request) are generated in a tree structure that reflects the GraphQL resolution tree. This sounds great in theory, but in practice it generates this kind of traces, where child spans start when their parent spans end.

 v root span                             | ----------------------------------------
 |  v resolve thing                      | ----------
 |  |  v resolve thing.field1            |           ----------
 |  |  \-    resolve thing.field1.nested |                     --------------------
 \- \-   resolve thing.field2            |           --------------------

Although it may look fine at a glance, it has issues:

• Collapsing a span with child spans in a trace viewer (maybe some of them handle this better?) results in some amount of time seemingly unaccounted for in the root span. E.g.:

   v root span                             | ----------------------------------------
   |  v resolve thing                      | ----------
   |  |  > resolve thing.field1            |           ----------
   \- \-   resolve thing.field2            |           --------------------
  

   v root span                             | ----------------------------------------
   \- > resolve thing                      | ----------
  

• Critical path in viewers like Jaeger does not generate properly

Short description of the changes

The solution is to never have a span end before all its child spans have ended. I can imagine two ways to achieve this:

1. Add an extra layer of spans for the tree structure, and put the "resolve" spans under their tree structure node.

    v root span                    | ----------------------------------------
    |  v thing                     | ----------------------------------------
    |  |    resolve                | ----------
    |  |  v thing.field1           |           ------------------------------
    |  |  |    resolve             |           ----------
    |  |  |  v thing.field1.nested |                     --------------------
    |  |  \- \-   resolve          |                     --------------------
    |  |  v thing.field2           |           --------------------
    \- \- \-   resolve             |           --------------------
   

2. Put all "resolve" spans under the same parent.

   v root span                      | ----------------------------------------
   |    resolve thing               | ----------
   |    resolve thing.field1        |           ----------
   |    resolve thing.field2        |           --------------------
   \-   resolve thing.field1.nested |                     --------------------
   

This PR implements option 2. for simplicity, although I am open to work toward option 1. if preferred.