Support sampling.priority tag to control span sampling flag

See https://github.com/opentracing/specification/blob/master/semantic_conventions.md

lib/jaeger/client/span.rb

@@ -23,8 +23,10 @@ def initialize(context, operation_name, reporter, start_time: Time.now, referenc
         @reporter = reporter
         @start_time = start_time
         @references = references
-        @tags = tags.map { |key, value| ThriftTagBuilder.build(key, value) }
+        @tags = []
         @logs = []
+
+        tags.each { |key, value| set_tag(key, value) }
       end
 
       # Set a tag value on this span
@@ -33,8 +35,21 @@ def initialize(context, operation_name, reporter, start_time: Time.now, referenc
       # @param value [String, Numeric, Boolean] the value of the tag. If it's not
       # a String, Numeric, or Boolean it will be encoded with to_s
       def set_tag(key, value)
+        if key == 'sampling.priority'
+          if value.to_i > 0
+            return self if @context.debug?
+
+            @context.flags = @context.flags | SpanContext::Flags::SAMPLED | SpanContext::Flags::DEBUG
+          else
+            @context.flags = @context.flags & ~SpanContext::Flags::SAMPLED
+          end
+          return self
+        end
+
         # Using Thrift::Tag to avoid unnecessary memory allocations
         @tags << ThriftTagBuilder.build(key, value)
+
+        self
       end
 
       # Set a baggage item on the span

lib/jaeger/client/span_context.rb

@@ -28,6 +28,7 @@ def self.create_from_parent_context(span_context)
       end
 
       attr_reader :span_id, :parent_id, :trace_id, :baggage, :flags
+      attr_writer :flags
 
       def initialize(span_id:, parent_id: 0, trace_id:, flags:, baggage: {})
         @span_id = span_id

spec/jaeger/client/span_spec.rb

@@ -45,4 +45,23 @@
     span.set_baggage_item('foo', 'baz')
     expect(span.get_baggage_item('foo')).to eq('baz')
   end
+
+  describe '#set_tag' do
+    let(:span_context) { Jaeger::Client::SpanContext.create_parent_context }
+    let(:span) { described_class.new(span_context, 'operation_name', nil) }
+
+    context 'when sampling.priority' do
+      it 'sets debug flag to true when sampling.priority is greater than 0' do
+        span.set_tag('sampling.priority', 1)
+        expect(span.context.debug?).to eq(true)
+        expect(span.context.sampled?).to eq(true)
+      end
+
+      it 'sets sampled flag to false when sampling.priority is 0' do
+        span.set_tag('sampling.priority', 0)
+        expect(span.context.debug?).to eq(false)
+        expect(span.context.sampled?).to eq(false)
+      end
+    end
+  end
 end