Improve compatibility with Faraday v1

Fixes open-telemetry#1031

Faraday >= 1 changes where the #adapter method gets called, affecting
the RackBuilder patch in this gem.

Without this change, the tracer middleware gets added to the beginning
of the stack instead of the end, and explicitly adding the middleware
would cause it to get added twice.

This change patches `Connection#initialize` for Faraday >= 1 to preserve
the behavior we had with Faraday < 1. This is similar to the dd-trace-rb
change made in DataDog/dd-trace-rb#906.

There is some followup work here to explore moving the middleware to the
beginning of the stack instead of the end, but we can work on that
separately.

instrumentation/faraday/lib/opentelemetry/instrumentation/faraday/instrumentation.rb

@@ -27,6 +27,7 @@ class Instrumentation < OpenTelemetry::Instrumentation::Base
 
         def require_dependencies
           require_relative 'middlewares/tracer_middleware'
+          require_relative 'patches/connection'
           require_relative 'patches/rack_builder'
         end
 
@@ -37,7 +38,11 @@ def register_tracer_middleware
         end
 
         def use_middleware_by_default
-          ::Faraday::RackBuilder.prepend(Patches::RackBuilder)
+          if Gem::Version.new(::Faraday::VERSION) >= Gem::Version.new('1')
+            ::Faraday::Connection.prepend(Patches::Connection)
+          else
+            ::Faraday::RackBuilder.prepend(Patches::RackBuilder)
+          end
         end
       end
     end

instrumentation/faraday/lib/opentelemetry/instrumentation/faraday/patches/connection.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+# Copyright The OpenTelemetry Authors
+#
+# SPDX-License-Identifier: Apache-2.0
+
+module OpenTelemetry
+  module Instrumentation
+    module Faraday
+      module Patches
+        # Module to be prepended to force Faraday to use the middleware by
+        # default so the user doesn't have to call `use` for every connection.
+        module Connection
+          # Wraps Faraday::Connection#initialize:
+          # https://github.com/lostisland/faraday/blob/ff9dc1d1219a1bbdba95a9a4cf5d135b97247ee2/lib/faraday/connection.rb#L62-L92
+          def initialize(*args)
+            super.tap do
+              use(:open_telemetry) unless builder.handlers.any? do |handler|
+                handler.klass == Middlewares::TracerMiddleware
+              end
+            end
+          end
+        end
+      end
+    end
+  end
+end

instrumentation/faraday/test/opentelemetry/instrumentation/faraday/middlewares/tracer_middleware_test.rb

@@ -198,5 +198,18 @@
         _(span.status.code).must_equal OpenTelemetry::Trace::Status::ERROR
       end
     end
+
+    describe 'when explicitly adding the tracer middleware' do
+      let(:client) do
+        Faraday.new do |builder|
+          builder.use :open_telemetry
+        end
+      end
+
+      it 'only adds the middleware once' do
+        tracers = client.builder.handlers.count(OpenTelemetry::Instrumentation::Faraday::Middlewares::TracerMiddleware)
+        _(tracers).must_equal 1
+      end
+    end
   end
 end