Upgrade gitlab-labkit to 0.3.0

This matches the version used in Gitaly, bumps the gRPC version to be
aligned with GitLab CE/EE, and adds gRPC interceptor support:
https://gitlab.com/gitlab-org/labkit-ruby/compare/v0.2.0...v0.3.0

Gemfile

@@ -281,7 +281,7 @@ gem 'sentry-raven', '~> 2.9'
 gem 'premailer-rails', '~> 1.9.7'
 # LabKit: Tracing and Correlation
-gem 'gitlab-labkit', '~> 0.2.0'
+gem 'gitlab-labkit', '~> 0.3.0'
 # I18n
 gem 'ruby_parser', '~> 3.8', require: false

Gemfile.lock

@@ -306,10 +306,10 @@ GEM
     github-markup (1.7.0)
     gitlab-default_value_for (3.1.1)
       activerecord (>= 3.2.0, < 6.0)
-    gitlab-labkit (0.2.0)
+    gitlab-labkit (0.3.0)
       actionpack (~> 5)
       activesupport (~> 5)
-      grpc (~> 1.15)
+      grpc (~> 1.19.0)
       jaeger-client (~> 0.10)
       opentracing (~> 0.4)
     gitlab-markup (1.7.0)
@@ -1094,7 +1094,7 @@ DEPENDENCIES
   gitaly-proto (~> 1.32.0)
   github-markup (~> 1.7.0)
   gitlab-default_value_for (~> 3.1.1)
-  gitlab-labkit (~> 0.2.0)
+  gitlab-labkit (~> 0.3.0)
   gitlab-markup (~> 1.7.0)
   gitlab-sidekiq-fetcher (~> 0.4.0)
   gitlab-styles (~> 2.7)