Datadog tracing - Implement unified tagging `graphql.*`

[TonyCTHsu]

Why

Datadog is working on unified tagging initiative, in order to provide unified experience across different language implementation

https://github.com/DataDog/dd-trace-rb/pull/2278

What

graphql.source

A representation of source input to GraphQL.

candidates: graphql.parse, graphql.validate, and graphql.execute

graphql.operation.type

The operation type is either query, mutation, or subscription and describes what type of operation intending to do.

candidate: graphql.execute

graphql.operation.type

The operation name is a meaningful and explicit name for operation. It is only required in multi-operation documents, but its use is encouraged because it is very helpful for debugging and server-side logging.

candidate: graphql.execute