Use to_s on variables in LoggerTracer

- It's likely this was causing an issue with Kibana
  picking up our logs
- See https://gitlab.com/gitlab-org/gitlab/-/issues/331697#note_722162232

lib/gitlab/graphql/tracers/logger_tracer.rb

@@ -45,14 +45,12 @@ module Gitlab
           ::Gitlab::GraphqlLogger.info(info)
         end
 
-        def query_variables_for_logging(query)
-          clean_variables(query.provided_variables)
-        end
-
         def clean_variables(variables)
-          ActiveSupport::ParameterFilter
+          filtered = ActiveSupport::ParameterFilter
             .new(::Rails.application.config.filter_parameters)
             .filter(variables)
+
+          filtered&.to_s
         end
       end
     end

spec/lib/gitlab/graphql/tracers/logger_tracer_spec.rb

@@ -44,7 +44,7 @@ RSpec.describe Gitlab::Graphql::Tracers::LoggerTracer do
       query_fingerprint: query.fingerprint,
       query_string: query_string,
       trace_type: "execute_query",
-      variables: variables
+      variables: variables.to_s
     })
 
     dummy_schema.execute(query_string, variables: variables)

spec/requests/api/graphql_spec.rb

@@ -32,7 +32,7 @@ RSpec.describe 'GraphQL' do
           # operation_fingerprint starts with operation name
           operation_fingerprint: %r{^anonymous\/},
           is_mutation: false,
-          variables: variables,
+          variables: variables.to_s,
           query_string: query
         }
       end