…se it.

This commit introduces a `plugin` (named) export from the `apollo-tracing`
package, which uses the new "plugin" hooks rather than the previous
implementation (exported as `TracingExtension`) which uses the
soon-to-be-deprecated "extensions" hooks.  The functionality is intended to
be identical, in spirit.

Since the delta of the commits was otherwise confusing, I've left the
`TracingExtension` present and exported and will remove it in a subsequent
commit.

Briefly summarizing what the necessary changes were:

1. We no longer use a class instance to house the extension, which was
   necessitated by the `graphql-extensions` API.  This means that uses of
   `this` have been replaced with function scoped variables by the same name.
2. The logic which actually does the formatting (previously handled by the
   `format` method in `graphql-extension`, now takes place within the plugin
   API's `willSendResponse` method.

This follows-up 672fa09 and removes the no-longer used/necessary
`TracingExtension` extension which uses the soon-to-be-deprecated
`graphql-extensions` API, now that it has been replaced with an
implementation that uses the plugin API.

This package should never have already set it itself prior to this point,
but the circumstance exists that something (anything) else could have
already used the `extensions` sink to attach a value.

This guards against that.

…ernix/migrate-tracing-to-extension-api

…abernix/migrate-tracing-to-extension-api

…abernix/migrate-tracing-to-extension-api

This reverts the public messaging about `tracing` deprecation in commit
98bae44, for now, until we can discuss its
fate a bit more, and how we intend on making up for that.

…abernix/migrate-tracing-to-extension-api

…abernix/migrate-tracing-to-extension-api

…abernix/migrate-tracing-to-extension-api

Implements pattern gained by a926b7e in #3988.

Ref: #3998

…rate-tracing-to-extension-api