[core] Add Span.set_tag('force.keep') support

lib/ddtrace/span.rb

@@ -3,6 +3,7 @@
 
 require 'ddtrace/utils'
 require 'ddtrace/ext/errors'
+require 'ddtrace/ext/priority'
 
 module Datadog
   # Represents a logical unit of work in the system. Each trace consists of one or more spans.
@@ -68,7 +69,15 @@ def initialize(tracer, name, options = {})
     # must be strings. A valid example is:
     #
     #   span.set_tag('http.method', request.method)
-    def set_tag(key, value)
+    def set_tag(key, value = nil)
+      # 'force.keep' is an alias for setting the sampling priority to USER_KEEP
+      if key == 'force.keep'
+        return if @context.nil?
+
+        @context.sampling_priority = Datadog::Ext::Priority::USER_KEEP
+        return
+      end
+
       @meta[key] = value.to_s
     rescue StandardError => e
       Datadog::Tracer.log.debug("Unable to set the tag #{key}, ignoring it. Caused by: #{e}")

spec/ddtrace/span_spec.rb

@@ -0,0 +1,19 @@
+require 'spec_helper'
+
+require 'ddtrace'
+
+RSpec.describe Datadog::Span do
+  subject(:span) { described_class.new(tracer, 'test.span', options) }
+  let(:options) { { context: context } }
+  let(:tracer) { get_test_tracer }
+  let(:context) { Datadog::Context.new }
+
+  describe '#set_tag' do
+    context 'with \'force.keep\' key' do
+      before(:each) { span.set_tag('force.keep') }
+
+      it { expect(span.get_tag('force.keep')).to be_nil }
+      it { expect(context.sampling_priority).to eq(2) }
+    end
+  end
+end