Test gaps and Metrics.Poller supervision

[gsmlg]

Test gaps

• No tests for Caddy.Server.Embedded (only test/caddy/server/external_test.exs exists)
• No tests for Caddy.Metrics.Parser (only poller_test.exs)
• No transport error-path tests for Caddy.Admin.Request against a real unix socket

Metrics.Poller supervision

config :caddy, metrics_enabled: true and metrics_interval are documented in Caddy.Metrics.Poller, but the poller is not part of the supervision tree — it is manual-start only. If it's meant to be config-driven, Caddy.Supervisor should conditionally start it; otherwise the config options should be removed from the docs.