Fix AssertionError caused by the DatadogTracingExtension whenever the query is unavailable.
The bug in question was reported by issue #3150.
The datadog extension would throw an AssertionError whenever there was no query available. This could happen if,
for example, a user POSTed something to /graphql with a JSON that doesn't contain a query field as per the
GraphQL spec.
The fix consists of adding query_missing to the operation_type tag, and also adding query_missing to the resource name.
It also makes it easier to look for logs of users making invalid queries by searching for query_missing in Datadog.
Releases contributed by @serramatutu via #3483