Add observatory.md (en)

docs/en/config/observatory.md

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+# Connection Monitoring
+
+The connection monitoring component uses HTTPing to detect the connection
+status of outbound proxies. The monitoring results can be used by other
+components, such as load balancers. There are currently two options:
+[observatory](#observatoryobject) (background connection monitoring) and
+[burstObservatory](#burstobservatoryobject) (concurrent connection monitoring).
+You can choose one of them as needed.
+
+## ObservatoryObject
+```json
+{
+  "subjectSelector":[
+    "outbound"
+  ],
+  "probeUrl": "https://www.google.com/generate_204",
+  "probeInterval": "10s",
+  "enableConcurrency": false
+}
+```
+
+> `subjectSelector`: \[ string \]
+
+An array of strings, where each string is used to match the prefix of outbound proxy identifiers. Among the following outbound proxy identifiers: `["a", "ab", "c", "ba"]`, `"subjectSelector": ["a"]` will match `["a", "ab"]`.
+
+> `probeUrl`: string
+
+The URL used to detect the connection status of the outbound proxy.
+
+> `probeInterval`: string
+
+The interval at which probes are initiated. The time format is a number followed by a unit, such as `"10s"`, `"2h45m"`. Supported time units include `ns`, `us`, `ms`, `s`, `m`, `h`, corresponding to nanoseconds, microseconds, milliseconds, seconds, minutes, and hours, respectively.
+
+> `enableConcurrency`: true | false
+
+- `true`: Concurrently probe all matching outbound proxies, then pause for the time set by `probeInterval`.
+- `false`: Probe each matching outbound proxy one by one, pausing for the time set by `probeInterval` after probing each one.
+
+## BurstObservatoryObject
+```json
+{
+  "subjectSelector":[
+    "outbound"
+  ],
+  "pingConfig": {}
+}
+```
+
+> `subjectSelector`: \[ string \]
+
+An array of strings, where each string is used to match the prefix of outbound proxy identifiers. Among the following outbound proxy identifiers: `["a", "ab", "c", "ba"]`, `"subjectSelector": ["a"]` will match `["a", "ab"]`.
+
+> `pingConfig`: [PingConfigObject](#PingConfigObject)
+
+
+### PingConfigObject
+```json
+{
+  "destination": "https://connectivitycheck.gstatic.com/generate_204",
+  "connectivity": "",
+  "interval": "1h",
+  "sampling": 3,
+  "timeout": "30s"
+}
+```
+
+> `destination`: string
+
+The URL used to detect the connection status of the outbound proxy. This URL should return an HTTP 204 success status code.
+
+> `connectivity`: string
+
+The URL used to check local network connectivity. An empty string means that local network connectivity is not checked.
+
+> `interval`: string
+
+Within the specified time, probe all matching outbound proxies, probing each proxy `sampling + 1` times. The time format is a number followed by a unit, such as `"10s"`, `"2h45m"`. Supported time units include `ns`, `us`, `ms`, `s`, `m`, `h`, corresponding to nanoseconds, microseconds, milliseconds, seconds, minutes, and hours, respectively.
+
+> `sampling`: number
+
+The number of recent probe results to retain.
+
+> `timeout`: string
+
+The probe timeout period. The format is the same as the `interval` above.