Metrics API: Common Metrics (partially done)

[AlvaroVega]

https://docs.google.com/document/d/1RzoYyAON_6VFKCrpmh-ATUCfFabkrGN9i9AnMpnVfdg/edit#heading=h.zay0ixgoh60l

This issue is about implementing the basic API metrics operations, along with the first batch of counters (the common ones for all the IoTP components).

Operations to implement:

GET <host:port>/admin/metrics
DELETE <host:port>/admin/metrics
GET <host:port>/admin/metrics?reset=true (maybe, yet under discussion)

Syntax:

{
    "service": {
        "service1": {
            "subservs": {
                "subserv1": {},
                "subserv2": {},
                "subserv3": {}
            },
            "sum": {}
        },
        "service2": {
            "subservs": {
                "subserv1": {},
                "subserv2": {},
                "subserv3": {}
            },
            "sum": {}
        },
        "service3": {
            "subservs": {
                "subserv1": {},
                "subserv2": {},
                "subserv3": {}
            },
            "sum": {}
        }
    },
    "sum": {
        "subservs": {
            "subserv1": {},
            "subserv2": {},
            "subserv3": {}
        },
        "sum": {}
    }
}

Metrics to implement:

• incomingTransactions: number of requests consumed by the component.
• incomingTransactionRequestSize: total size (bytes) in requests associated to incoming transactions (“in” from the point of view of the component).
• incomingTransactionResponseSize: total size (bytes) in responses associated to incoming transactions (“out” from the point of view of the component).
• incomingTransacionError: number of incoming transactions resulting in error.
• serviceTime: average time to serve a transaction.
• outgoingTransactions: number of requests sent by the component.
• outgoingTransactionRequestSize: total size (bytes) in requests associated to outgoing transactions (“out” from the point of view of the component).
• outgoingTransactionResponseSize: total size (bytes) in responses associated to outgoing transactions (“in” from the point of view of the component).
• outgoingTransacionError: number of outgoing transactions resulting in error.

Note: all incoming/outgoing channels count for the above metrics.