Fix: Add Sinatra documentation & small refactor

Author: hannahramadan

instrumentation/sinatra/README.md

@@ -72,3 +72,21 @@ The `opentelemetry-instrumentation-sinatra` gem is distributed under the Apache
 [community-meetings]: https://github.com/open-telemetry/community#community-meetings
 [slack-channel]: https://cloud-native.slack.com/archives/C01NWKKMKMY
 [discussions-url]: https://github.com/open-telemetry/opentelemetry-ruby/discussions
+
+## HTTP semantic convention stability
+
+In the OpenTelemetry ecosystem, HTTP semantic conventions have now reached a stable state. However, the initial Rack instrumentation was introduced before this stability was achieved, which resulted in HTTP attributes being based on an older version of the semantic conventions. 
+
+To facilitate the migration to stable semantic conventions, you can use the `OTEL_SEMCONV_STABILITY_OPT_IN` environment variable. This variable allows you to opt-in to the new stable conventions, ensuring compatibility and future-proofing your instrumentation.
+
+Sinatra instrumentaion installs Rack middleware, but the middleware version it installs depends on which `OTEL_SEMCONV_STABILITY_OPT_IN` environment variable is set.
+
+When setting the value for `OTEL_SEMCONV_STABILITY_OPT_IN`, you can specify which conventions you wish to adopt:
+
+- `http` - Emits the stable HTTP and networking conventions and ceases emitting the old conventions previously emitted by the instrumentation.
+- `http/dup` - Emits both the old and stable HTTP and networking conventions, enabling a phased rollout of the stable semantic conventions.
+- Default behavior (in the absence of either value) is to continue emitting the old HTTP and networking conventions the instrumentation previously emitted.
+
+During the transition from old to stable conventions, Rack instrumentation code comes in three patch versions: `dup`, `old`, and `stable`. These versions are identical except for the attributes they send. Any changes to Rack instrumentation should consider all three patches.
+
+For additional information on migration, please refer to our [documentation](https://opentelemetry.io/docs/specs/semconv/non-normative/http-migration/).

instrumentation/sinatra/example/config.ru

@@ -47,5 +47,9 @@ class App < Sinatra::Base
   end
 end
 
+# Rack instrumentation is moving through the process of migrating to the new HTTP semantic
+# conventions. In this example, we will use the old HTTP conventions by patching the Rack
+# middleware that uses the old conventions. See README: HTTP Semantic Conventions for more
+# information.
 use(*OpenTelemetry::Instrumentation::Rack::Instrumentation.instance.middleware_args_old)
 run App

instrumentation/sinatra/lib/opentelemetry/instrumentation/sinatra/instrumentation.rb

@@ -51,10 +51,10 @@ class Instrumentation < OpenTelemetry::Instrumentation::Base
         end
 
         def install_middleware(app)
-          stability_opt_in = ENV.fetch('OTEL_SEMCONV_STABILITY_OPT_IN', '')
-          values = stability_opt_in.split(',').map(&:strip)
-
           if config[:install_rack]
+            stability_opt_in = ENV.fetch('OTEL_SEMCONV_STABILITY_OPT_IN', '')
+            values = stability_opt_in.split(',').map(&:strip)
+
             if values.include?('http/dup')
               app.use(*OpenTelemetry::Instrumentation::Rack::Instrumentation.instance.middleware_args_dup)
             elsif values.include?('http')