Fix NoMethodError in sinatra integration when Tracer middleware is missing

This fixes a regression introduced in version 0.52.0 by #1628.

When a Sinatra modular application being traced is missing the
`Datadog::Contrib::Sinatra::Tracer` middleware
(as documented in
<https://docs.datadoghq.com/tracing/setup_overview/setup/ruby/#sinatra>),
no Sinatra request span is ever created (only a route span).

Because we didn't properly account for this in `#datadog_span`, the
following `NoMethodError` was triggered:

```
NoMethodError: undefined method `[]' for nil:NilClass
  dd-trace-rb/lib/ddtrace/contrib/sinatra/env.rb:13:in `datadog_span'
  dd-trace-rb/lib/ddtrace/contrib/sinatra/tracer.rb:122:in `block in route_eval'
  dd-trace-rb/lib/ddtrace/tracer.rb:283:in `trace'
  dd-trace-rb/lib/ddtrace/contrib/sinatra/tracer.rb:105:in `route_eval'
  ruby-2.7.4/gems/sinatra-2.1.0/lib/sinatra/base.rb:1013:in `block (2 levels) in route!'
```

I suspect this is the same issue as was reported in #1643.

Because this configuration is actually not desirable -- spans reported
will be missing crucial request information, as discussed in #1643 --
I've also added a warning to hopefully steer users in the right
direction.

Author: ivoanjo

lib/ddtrace/contrib/sinatra/env.rb

@@ -10,7 +10,8 @@ module Env
         module_function
 
         def datadog_span(env, app)
-          env[Ext::RACK_ENV_REQUEST_SPAN][app]
+          request_span = env[Ext::RACK_ENV_REQUEST_SPAN]
+          request_span && request_span[app]
         end
 
         def set_datadog_span(env, app, span)

lib/ddtrace/contrib/sinatra/tracer.rb

@@ -5,7 +5,7 @@
 require 'ddtrace/ext/errors'
 require 'ddtrace/ext/http'
 require 'ddtrace/propagation/http_propagator'
-
+require 'ddtrace/utils/only_once'
 require 'ddtrace/contrib/sinatra/ext'
 require 'ddtrace/contrib/sinatra/tracer_middleware'
 require 'ddtrace/contrib/sinatra/env'
@@ -77,6 +77,9 @@ def self.registered(app)
 
         # Method overrides for Sinatra::Base
         module Base
+          MISSING_REQUEST_SPAN_ONLY_ONCE = Datadog::Utils::OnlyOnce.new
+          private_constant :MISSING_REQUEST_SPAN_ONLY_ONCE
+
           def render(engine, data, *)
             tracer = Datadog.configuration[:sinatra][:tracer]
             return super unless tracer.enabled
@@ -121,8 +124,18 @@ def route_eval
                 else
                   Sinatra::Env.datadog_span(env, self.class)
                 end
-              if sinatra_request_span # DEV: Is it possible for sinatra_request_span to ever be nil here?
+              if sinatra_request_span
                 sinatra_request_span.resource = span.resource
+              else
+                MISSING_REQUEST_SPAN_ONLY_ONCE.run do
+                  Datadog.logger.warn do
+                    'Sinatra integration is misconfigured, reported traces will be missing request metadata ' \
+                    'such as path and HTTP status code. ' \
+                    'Did you forget to add `register Datadog::Contrib::Sinatra::Tracer` to your ' \
+                    '`Sinatra::Base` subclass? ' \
+                    'See <https://docs.datadoghq.com/tracing/setup_overview/setup/ruby/#sinatra> for more details.'
+                  end
+                end
               end
 
               Contrib::Analytics.set_measured(span)

spec/ddtrace/contrib/sinatra/tracer_spec.rb

@@ -609,6 +609,35 @@
         end
       end
     end
+
+    context 'when modular app does not register the Datadog::Contrib::Sinatra::Tracer middleware' do
+      let(:sinatra_app) do
+        sinatra_routes = self.sinatra_routes
+        stub_const('App', Class.new(Sinatra::Base) do
+          instance_exec(&sinatra_routes)
+        end)
+      end
+
+      subject(:response) { get url }
+
+      before do
+        allow(Datadog.logger).to receive(:warn)
+        Datadog::Contrib::Sinatra::Tracer::Base
+          .const_get('MISSING_REQUEST_SPAN_ONLY_ONCE').send(:reset_ran_once_state_for_tests)
+      end
+
+      it { is_expected.to be_ok }
+
+      it 'logs a warning' do
+        expect(Datadog.logger).to receive(:warn) do |&message|
+          expect(message.call).to include 'Sinatra integration is misconfigured'
+        end
+
+        # NOTE: We actually need to check that the request finished ok, as sinatra may otherwise "swallow" an RSpec
+        # exception and thus the test will appear to pass because the RSpec exception doesn't propagate out
+        is_expected.to be_ok
+      end
+    end
   end
 
   RSpec::Matchers.define :be_request_span do |opts = {}|