Add nginx_upstream_check input plugin (#4303)

plugins/inputs/nginx_upstream_check/README.md

@@ -0,0 +1,75 @@
+# Telegraf Plugin: Nginx_upstream_check
+
+Read the status output of the nginx_upstream_check (https://github.com/yaoweibin/nginx_upstream_check_module).
+This module can periodically check the servers in the Nginx's upstream with configured request and interval to determine
+if the server is still available. If checks are failed the server is marked as "down" and will not receive any requests
+until the check will pass and a server will be marked as "up" again.
+
+The status page displays the current status of all upstreams and servers as well as number of the failed and successful
+checks. This information can be exported in JSON format and parsed by this input.
+
+### Configuration:
+
+```
+  ## An URL where Nginx Upstream check module is enabled
+  ## It should be set to return a JSON formatted response
+  url = "http://127.0.0.1/status?format=json"
+
+  ## HTTP method
+  # method = "GET"
+
+  ## Optional HTTP headers
+  # headers = {"X-Special-Header" = "Special-Value"}
+
+  ## Override HTTP "Host" header
+  # host_header = "check.example.com"
+
+  ## Timeout for HTTP requests
+  timeout = "5s"
+
+  ## Optional 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
+```
+
+### Measurements & Fields:
+
+- Measurement
+    - fall (The number of failed server check attempts, counter)
+    - rise (The number of successful server check attempts, counter)
+    - status (The reporter server status as a string)
+    - status_code (The server status code. 1 - up, 2 - down, 0 - other)
+
+The "status_code" field most likely will be the most useful one because it allows you to determine the current
+state of every server and, possible, add some monitoring to watch over it. InfluxDB can use string values and the
+"status" field can be used instead, but for most other monitoring solutions the integer code will be appropriate.
+
+### Tags:
+
+- All measurements have the following tags:
+    - name (The hostname or IP of the upstream server)
+    - port (The alternative check port, 0 if the default one is used)
+    - type (The check type, http/tcp)
+    - upstream (The name of the upstream block in the Nginx configuration)
+    - url (The status url used by telegraf)
+
+### Example Output:
+
+When run with:
+```
+./telegraf --config telegraf.conf --input-filter nginx_upstream_check --test
+```
+
+It produces:
+```
+* Plugin: nginx_upstream_check, Collection 1
+> nginx_upstream_check,host=node1,name=192.168.0.1:8080,port=0,type=http,upstream=my_backends,url=http://127.0.0.1:80/status?format\=json fall=0i,rise=100i,status="up",status_code=1i 1529088524000000000
+> nginx_upstream_check,host=node2,name=192.168.0.2:8080,port=0,type=http,upstream=my_backends,url=http://127.0.0.1:80/status?format\=json fall=100i,rise=0i,status="down",status_code=2i 1529088524000000000
+```