Wire fork safety into native transport via AtForkMonkeyPatch

Register a `:child` callback that calls `_native_after_fork_in_child`
on the exporter to recreate the tokio runtime in forked child
processes. Without this, the Rust runtime is dead after fork and
subsequent send calls would hang or fail.

The `AtForkMonkeyPatch` only supports `:child` stage, so
`before_fork` and `after_fork_in_parent` are not called. The child
path is the critical one: it creates a fresh runtime regardless of
whether the parent was prepared.

Author: lloeki

lib/datadog/tracing/transport/native.rb

@@ -77,6 +77,15 @@ def initialize(agent_settings:, logger: Datadog.logger)
               service: service,
               version: version
             )
+
+            # In forked child processes the tokio runtime is dead.
+            # Recreate it so the exporter can send traces again.
+            exporter = @exporter
+            Core::Utils::AtForkMonkeyPatch.at_fork(:child) do
+              exporter._native_after_fork_in_child
+            rescue => e
+              Datadog.logger.warn { "Native transport after-fork reset failed: #{e}" }
+            end
           end
 
           # Send a list of traces to the agent.