feat: add a healthcheck endpoint to frontend-ruby (#1008)

## Which problem is this PR solving?

- Demonstrate how to ignore endpoints in OTel Ruby instrumentation,
particular use case being chatty healthcheck endpoints.

## Short description of the changes

- adds a HEALTHCHECK command to the Dockerfile so that Docker will nag
the healthcheck endpoint
- adds OTel instrumentation config to ignore healthcheck endpoint
(doesn't generate traces for requests to /up)

ruby/frontend/Dockerfile

@@ -7,4 +7,6 @@ COPY frontend.ru /myapp
 COPY o11y_wrapper.rb /myapp
 
 EXPOSE 7777
+
+HEALTHCHECK --interval=30s --timeout=30s --start-period=5s --retries=3 CMD [ "curl", "--fail", "http://localhost:7777/up" ]
 CMD [ "bundle", "exec", "rackup", "frontend.ru", "--server", "puma", "--host", "0.0.0.0"]

ruby/frontend/frontend.ru

@@ -14,8 +14,13 @@ begin
   OpenTelemetry::SDK.configure do |c|
     c.service_name = ENV['SERVICE_NAME'] || "frontend-ruby"
 
-    # enable all auto-instrumentation available
-    c.use_all()
+    # enable all instrumentation libraries installed in the Gemfile
+    # and configure some
+    c.use_all({
+      'OpenTelemetry::Instrumentation::Rack' => {
+        untraced_endpoints: ['/up'], # don't create traces for healthchecks
+      },
+    })
 
     # add the Baggage and CarryOn processors to thepipeline
     c.add_span_processor(O11yWrapper::BaggageSpanProcessor.new)
@@ -49,10 +54,17 @@ class FrontendGreetingApp < Rails::Application
   Rails.logger  = config.logger
 
   routes.append do
+    get "/up" => "health#show"
     get "/greeting" => "greetings#index"
   end
 end
 
+class HealthController < ActionController::Base
+  def show
+    render plain: "ok"
+  end
+end
+
 class GreetingsController < ActionController::Base
   around_action :context_for_named_visitor