[distributed_headers] Accept signed integers as IDs.

This is what dd-trace-js sends over the wire:
https://github.com/DataDog/dd-trace-js/blob/aa607860ea6bb57f2f69fa7ee72ed3ce25abfbf6/src/opentracing/propagation/text_map.js#L14-L15

It’s also what has been patched in e.g. dd-trace-go:
DataDog/dd-trace-go#326

lib/ddtrace/propagation/distributed_headers.rb

@@ -16,15 +16,11 @@ def valid?
     end
 
     def trace_id
-      value = header(HTTP_HEADER_TRACE_ID).to_i
-      return if value <= 0 || value >= Span::MAX_ID
-      value
+      id HTTP_HEADER_TRACE_ID
     end
 
     def parent_id
-      value = header(HTTP_HEADER_PARENT_ID).to_i
-      return if value <= 0 || value >= Span::MAX_ID
-      value
+      id HTTP_HEADER_PARENT_ID
     end
 
     def sampling_priority
@@ -44,5 +40,11 @@ def header(name)
 
       @env[rack_header]
     end
+
+    def id(header)
+      value = header(header).to_i
+      return if value == 0 || value >= Span::MAX_ID
+      value < 0 ? value + 0x1_0000_0000_0000_0000 : value
+    end
   end
 end

test/propagation/distributed_headers_test.rb

@@ -48,7 +48,7 @@ def test_trace_id
     test_cases = {
       { 'HTTP_X_DATADOG_TRACE_ID' => '123' } => 123,
       { 'HTTP_X_DATADOG_TRACE_ID' => '0' } => nil,
-      { 'HTTP_X_DATADOG_TRACE_ID' => '-1' } => nil,
+      { 'HTTP_X_DATADOG_TRACE_ID' => '-1' } => 18446744073709551615,
       { 'HTTP_X_DATADOG_TRACE_ID' => 'ooops' } => nil,
       { 'HTTP_X_DATADOG_TRACE_TYPO' => '1' } => nil,
       { 'HTTP_X_DATADOG_TRACE_ID' => Datadog::Span::MAX_ID.to_s } => nil,
@@ -69,7 +69,7 @@ def test_parent_id
     test_cases = {
       { 'HTTP_X_DATADOG_PARENT_ID' => '123' } => 123,
       { 'HTTP_X_DATADOG_PARENT_ID' => '0' } => nil,
-      { 'HTTP_X_DATADOG_PARENT_ID' => '-1' } => nil,
+      { 'HTTP_X_DATADOG_PARENT_ID' => '-1' } => 18446744073709551615,
       { 'HTTP_X_DATADOG_PARENT_ID' => 'ooops' } => nil,
       { 'HTTP_X_DATADOG_PARENT_TYPO' => '1' } => nil,
       { 'HTTP_X_DATADOG_PARENT_ID' => Datadog::Span::MAX_ID.to_s } => nil,