Metrics API: Common Metrics

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.

Start by locating the existing admin API entry point and the component's incoming and outgoing transaction channels. Implement and verify the GET and DELETE metrics operations, with the documented common counters and response structure; treat the reset=true behavior as unresolved until its discussion is settled.