Add Nginx monitoring to instrumentald

[janxious]

Straight from the source:

Active connections
    The current number of active client connections including Waiting connections. 
accepts
    The total number of accepted client connections. 
handled
    The total number of handled connections. Generally, the parameter value is the same as accepts unless some resource limits have been reached (for example, the worker_connections limit). 
requests
    The total number of client requests. 
Reading
    The current number of connections where nginx is reading the request header. 
Writing
    The current number of connections where nginx is writing the response back to the client. 
Waiting
    The current number of idle client connections waiting for a request.