Set 'changelog_uri' in gemspec to point to CHANGELOG.md

Changelog URLs have been valid [metadata](https://guides.rubygems.org/specification-reference/#metadata) since 2017 on RubyGems.org.
Setting `changelog_uri` will add a link "Changelog" link to the Rubygems page for ddtrace https://rubygems.org/gems/ddtrace/

ddtrace.gemspec

@@ -25,6 +25,7 @@ Gem::Specification.new do |spec|
 
   if spec.respond_to?(:metadata)
     spec.metadata['allowed_push_host'] = 'https://rubygems.org'
+    spec.metadata['changelog_uri'] = 'https://github.com/DataDog/dd-trace-rb/blob/master/CHANGELOG.md'
   else
     raise 'RubyGems 2.0 or newer is required to protect against public gem pushes.'
   end