Warn when enabling socket_telemetry on unsupported platform

The structs we depend on for collecting socket telemetry (`SOL_TCP` and
`TCP_INFO`) don't exist on all platforms. In such cases, these constants
won't be defined, and so we should WARN users and not enabled the
`socket_telemetry` option.

NOTE: On macOS, similar structs do exist: `Socket::IPPROTO_TCP` and
`Socket::TCP_CONNECTION_INFO`, respectively, but I couldn't figure out
the binary layout of the data, so couldn't unpack it. Maybe someone else
knows more about those details and can dig in at some point.

lib/puma/plugin/telemetry/config.rb

@@ -91,7 +91,14 @@ def enabled?
         end
 
         def socket_telemetry!
-          @socket_telemetry = true
+          # These structs are platform specific, and not available on macOS,
+          # for example. If they're undefined, then we cannot capture socket
+          # telemetry. We'll warn in that case.
+          if defined?(Socket::SOL_TCP) && defined?(Socket::TCP_INFO)
+            @socket_telemetry = true
+          else
+            warn("Cannot capture socket telemetry on this platform (#{RUBY_PLATFORM}); socket_telemetry is disabled.")
+          end
         end
 
         def socket_telemetry?

spec/integration/plugin_spec.rb

@@ -105,6 +105,10 @@ def make_request
         end
 
         it 'logs socket telemetry' do
+          unless defined?(Socket::SOL_TCP) && defined?(Socket::TCP_INFO)
+            skip("Socket::SOL_TCP not defined on #{RUBY_PLATFORM}")
+          end
+
           threads = Array.new(2) { make_request }
 
           sleep 0.1
@@ -123,7 +127,7 @@ def make_request
           possible_lines = ['queue.backlog=1 sockets.backlog=5',
                             'queue.backlog=0 sockets.backlog=6']
 
-          expect(possible_lines.include?(line)).to eq(true)
+          expect(possible_lines).to include(line)
 
           total = line.split.sum { |kv| kv.split('=').last.to_i }
           expect(total).to eq 6