Task/add health check to /iot/about /metrics and /ready

[AlvaroVega]

fix for #1763

• Update doc
• new config env vars: IOTA_HEALTH_CHECK_INTERVAL, IOTA_HEALTH_CHECK_TIMEOUT, IOTA_HEALTH_CHECK_DOWN_AFTER_FAILS
• Add Test

This produces:
curl -i -X GET http://localhost:4052/iot/about

{
"libVersion":"4.13.0-next",
"port":"4052",
"baseRoot":"/",
"version":"3.13.0-next",
"connections":{
  "contextBroker":{
     "ok":true,
     "configured":true,
     "url":"http://iot-orion:1026",
     "lastOk":"2026-02-27T13:28:53.484Z",
     "lastError":null,
     "latencyMs":7,
     "consecutiveFails":0},
  "iotagentManager":{"ok":true,"configured":true,"url":"http://iot-iotagent-manager:8082","lastOk":"2026-02-27T13:28:53.494Z","lastError":null,"latencyMs":17,"consecutiveFails":0},
  "mongodb":{"ok":true,"configured":true,"url":"mongodb://iot-mongo:27017/iotajson","lastOk":"2026-02-27T13:28:53.482Z","lastError":null,"latencyMs":5,"consecutiveFails":0},
  "mqtt":{"ok":true,"configured":true,"url":"mqtt://iot-mosquitto:1883","lastOk":"2026-02-27T13:28:53.485Z","lastError":null,"latencyMs":7,"consecutiveFails":0}
}
}

curl -i -X GET http://localhost:4052/ready
HTTP/1.1 200 OK
X-Powered-By: Express
Fiware-Correlator: 930a1b27-da53-4fe4-91fb-db26e1b5809f
Content-Type: application/json; charset=utf-8
Content-Length: 2
ETag: W/"2-vyGp6PvFo4RvsFtPoIWeCReyIC8"
Date: Fri, 06 Mar 2026 07:16:59 GMT
Connection: keep-alive
Keep-Alive: timeout=5

{}

[fgalan]

• The new 4 env vars should be mentioned
• The improvements in GET /metrics should be mentioned
• The new operation GET /ready should be mentioned

[AlvaroVega]

Done into 707e1ba