Deprecate tracer option

Author: TonyCTHsu

guides/queries/tracing.md

@@ -148,16 +148,10 @@ To add [Datadog](https://www.datadoghq.com) instrumentation:
 
 ```ruby
 class MySchema < GraphQL::Schema
-  use(GraphQL::Tracing::DataDogTracing, options)
+  use(GraphQL::Tracing::DataDogTracing)
 end
 ```
 
-You may provide `options` as a `Hash` with the following values:
-
-| Key | Description | Default |
-| --- | ----------- | ------- |
-| `tracer` | Tracer used to perform instrumentation. Usually you don't need to set this. | `Datadog::Tracing` |
-
 For more details about Datadog's tracing API, check out the [Ruby documentation](https://github.com/DataDog/dd-trace-rb/blob/master/docs/GettingStarted.md) or the [APM documentation](https://docs.datadoghq.com/tracing/) for more product information.
 
 ## Prometheus

lib/graphql/tracing/data_dog_trace.rb

@@ -3,6 +3,7 @@
 module GraphQL
   module Tracing
     module DataDogTrace
+      # @param tracer [#trace] Deprecated
       # @param analytics_enabled [Boolean] Deprecated
       # @param analytics_sample_rate [Float] Deprecated
       def initialize(tracer: nil, analytics_enabled: false, analytics_sample_rate: 1.0, service: nil, **rest)

lib/graphql/tracing/data_dog_tracing.rb

@@ -56,6 +56,7 @@ def prepare_span(key, data, span)
       def tracer
         default_tracer = defined?(Datadog::Tracing) ? Datadog::Tracing : Datadog.tracer
 
+        # [Deprecated] options[:tracer] will be removed in the future
         options.fetch(:tracer, default_tracer)
       end