Add pgbouncer input plugin (#3918)

plugins/inputs/pgbouncer/README.md

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+# PgBouncer plugin
+
+This PgBouncer plugin provides metrics for your PgBouncer load balancer.
+
+More information about the meaning of these metrics can be found in the [PgBouncer Documentation](https://pgbouncer.github.io/usage.html)
+
+## Configuration
+Specify address via a url matching:
+
+  `postgres://[pqgotest[:password]]@localhost[/dbname]?sslmode=[disable|verify-ca|verify-full]`
+
+All connection parameters are optional.
+
+Without the dbname parameter, the driver will default to a database with the same name as the user.
+This dbname is just for instantiating a connection with the server and doesn't restrict the databases we are trying to grab metrics for.
+
+### Configuration example
+```
+[[inputs.pgbouncer]]
+  address = "postgres://telegraf@localhost/pgbouncer"
+```