fix(faraday): Don't double instrument builder

sentry-ruby/lib/sentry/faraday.rb

@@ -14,10 +14,13 @@ def initialize(url = nil, options = nil)
 
         # Ensure that we attach instrumentation only if the adapter is not net/http
         # because if is is, then the net/http instrumentation will take care of it
-        if builder.adapter.name != "Faraday::Adapter::NetHttp"
+        if builder.adapter.name != "Faraday::Adapter::NetHttp" &&
+            !builder.locked? &&
+            !builder.instance_variable_defined?(:@sentry_patched)
           # Make sure that it's going to be the first middleware so that it can capture
           # the entire request processing involving other middlewares
           builder.insert(0, ::Faraday::Request::Instrumentation, name: OP_NAME, instrumenter: Instrumenter.new)
+          builder.instance_variable_set(:@sentry_patched, true)
         end
       end
     end

sentry-ruby/spec/sentry/faraday_spec.rb

@@ -20,6 +20,27 @@
     end
 
     context "with tracing enabled" do
+      it "does not insert instrumentation more than once when a builder is reused" do
+        stubs = Faraday::Adapter::Test::Stubs.new do |stub|
+          stub.get("/") { [200, {}, "ok"] }
+        end
+
+        builder = Faraday::RackBuilder.new do |connection|
+          connection.adapter :test, stubs
+        end
+
+        primary = Faraday.new("https://primary.example", builder: builder)
+        Faraday.new("https://secondary.example", builder: builder)
+
+        expect(builder.handlers.count { |handler| handler == Faraday::Request::Instrumentation }).to eq(1)
+
+        primary.get("/")
+
+        expect {
+          Faraday.new("https://fallback.example", builder: builder)
+        }.not_to raise_error
+      end
+
       let(:http) do
         Faraday.new(url) do |f|
           f.request :json