trace: Log graphql.variables rather than tag

According to the OT documentation tag values can be numeric types, strings, or
bools. The behavior of other tag value types is undefined at the OpenTracing
level. For example `github.com/lightstep/lightstep-tracer-go` generates error
events.

trace/trace.go

@@ -8,6 +8,7 @@ import (
 	"github.com/neelance/graphql-go/introspection"
 	opentracing "github.com/opentracing/opentracing-go"
 	"github.com/opentracing/opentracing-go/ext"
+	"github.com/opentracing/opentracing-go/log"
 )
 
 type TraceQueryFinishFunc func([]*errors.QueryError)
@@ -29,7 +30,7 @@ func (OpenTracingTracer) TraceQuery(ctx context.Context, queryString string, ope
 	}
 
 	if len(variables) != 0 {
-		span.SetTag("graphql.variables", variables)
+		span.LogFields(log.Object("graphql.variables", variables))
 	}
 
 	return spanCtx, func(errs []*errors.QueryError) {