Add synproxy input plugin (#5683)

plugins/inputs/synproxy/README.md

@@ -0,0 +1,49 @@
+# Synproxy Input Plugin
+
+The synproxy plugin gathers the synproxy counters. Synproxy is a Linux netfilter module used for SYN attack mitigation. 
+The use of synproxy is documented in `man iptables-extensions` under the SYNPROXY section.
+
+
+### Configuration
+
+The synproxy plugin does not need any configuration
+
+```toml
+[[inputs.synproxy]]
+  # no configuration
+```
+
+### Metrics
+
+The following synproxy counters are gathered
+
+- synproxy
+  - fields:
+    - cookie_invalid (uint32, packets, counter) - Invalid cookies
+    - cookie_retrans (uint32, packets, counter) - Cookies retransmitted
+    - cookie_valid (uint32, packets, counter) - Valid cookies
+    - entries (uint32, packets, counter) - Entries
+    - syn_received (uint32, packets, counter) - SYN received
+    - conn_reopened (uint32, packets, counter) - Connections reopened
+
+### Sample Queries
+
+Get the number of packets per 5 minutes for the measurement in the last hour from InfluxDB:
+```
+SELECT difference(last("cookie_invalid")) AS "cookie_invalid", difference(last("cookie_retrans")) AS "cookie_retrans", difference(last("cookie_valid")) AS "cookie_valid", difference(last("entries")) AS "entries", difference(last("syn_received")) AS "syn_received", difference(last("conn_reopened")) AS "conn_reopened" FROM synproxy WHERE time > NOW() - 1h GROUP BY time(5m) FILL(null);
+```
+
+### Troubleshooting
+
+Execute the following CLI command in Linux to test the synproxy counters:
+```
+cat /proc/net/stat/synproxy
+```
+
+### Example Output
+
+This section shows example output in Line Protocol format.
+
+```
+synproxy,host=Filter-GW01,rack=filter-node1 conn_reopened=0i,cookie_invalid=235i,cookie_retrans=0i,cookie_valid=8814i,entries=0i,syn_received=8742i 1549550634000000000
+```