Introduce a DiscoveryEntry struct and an optional DiscoveryPublisher
interface that providers can co-implement to declare things worth
probing by other checkers (TLS endpoints, HTTP probes, ACME challenges,
DNSSEC keys, ...) without having to re-parse raw observations.
DiscoveryEntry carries an opaque Payload: the SDK does not interpret
it. Producers and consumers agree on the Payload schema through a
separate contract (eg. a small shared Go package imported by
both) identified by the free-form Type string. This keeps the SDK
free of protocol-specific concepts; new entry families can appear
without touching it.
The /collect HTTP handler type-asserts the provider against
DiscoveryPublisher immediately after Collect and forwards the
resulting entries in ExternalCollectResponse.Entries.
Adds HealthResponse carrying inflight count, total requests, 1/5/15-min
EWMA load averages, uptime, and NumCPU so a scheduler can pick the least
busy worker. A background sampler updates the load averages every 5s,
stopped by a new idempotent Close method. Work endpoints (/collect,
/evaluate, /report) are wrapped with a trackWork middleware; /health
and /definition are excluded so polling traffic does not pollute the
signal.
Errors from provider.Collect() and json.Marshal were returned with HTTP
200, making failures invisible to monitoring, proxies, and clients that
check status codes. Return 500 Internal Server Error so HTTP-level
tooling can detect failures without parsing the response body.
