Add Kibana input plugin (#4585)

plugins/inputs/kibana/README.md

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+# Kibana input plugin
+
+The [kibana](https://www.elastic.co/) plugin queries Kibana status API to
+obtain the health status of Kibana and some useful metrics.
+
+This plugin has been tested and works on Kibana 6.x versions.
+
+### Configuration
+
+```toml
+[[inputs.kibana]]
+  ## specify a list of one or more Kibana servers
+  servers = ["http://localhost:5601"]
+
+  ## Timeout for HTTP requests
+  timeout = "5s"
+
+  ## HTTP Basic Auth credentials
+  # username = "username"
+  # password = "pa$$word"
+
+  ## Optional TLS Config
+  # tls_ca = "/etc/telegraf/ca.pem"
+  # tls_cert = "/etc/telegraf/cert.pem"
+  # tls_key = "/etc/telegraf/key.pem"
+  ## Use TLS but skip chain & host verification
+  # insecure_skip_verify = false
+```
+
+### Status mappings
+
+When reporting health (green/yellow/red), additional field `status_code`
+is reported. Field contains mapping from status:string to status_code:int
+with following rules:
+
+- `green` - 1
+- `yellow` - 2
+- `red` - 3
+- `unknown` - 0
+
+### Measurements & Fields
+
+- kibana
+  - status_code: integer (1, 2, 3, 0)
+  - heap_max_bytes: integer
+  - heap_used_bytes: integer
+  - uptime_ms: integer
+  - response_time_avg_ms: float
+  - response_time_max_ms: integer
+  - concurrent_connections: integer
+  - requests_per_sec: float
+
+### Tags
+
+- status (Kibana health: green, yellow, red)
+- name (Kibana reported name)
+- uuid (Kibana reported UUID)
+- version (Kibana version)
+- source (Kibana server hostname or IP)
+
+### Example Output
+
+kibana,host=myhost,name=my-kibana,source=localhost:5601,version=6.3.2 concurrent_connections=0i,heap_max_bytes=136478720i,heap_used_bytes=119231088i,response_time_avg_ms=0i,response_time_max_ms=0i,status="green",status_code=1i,uptime_ms=2187428019i 1534864502000000000