Update and add missing descriptions to ETCD module

[sayden]

Fixes #10239 and adds some missing descriptions.

I'm not sure if this module needs a bit of polishing. On one side we are using fields like average, current or standardDeviation that maybe should be omitted leaving only current. On the other side we have fields like etcd.self.recv.appendrequest.count that maybe should be called etcd.self.received.append_requests or a etcd.self.recv.pkgrate that maybe should be etcd.self.received.packages_rate.per_sec or etcd.store.compareanddelete.fail (etcd.store.compare_and_delete.fail)

By the way, I also saw that data.json files are not complete because the module uses a compose configuration without clustering. I'm not sure if this can just be updated by using a manually created cluster so that it shows all possible fields or it should be created in CI (or matched in the CI). For example, in the leader metricset we have this data.json file but we might have this:

{
    "@timestamp": "2017-10-12T08:05:34.853Z",
    "agent": {
        "hostname": "host.example.com",
        "name": "host.example.com"
    },
    "etcd": {
        "leader": {
            "followers": {
                "5a22bdba1efc5b4a": {
                    "latency": {
                        "average": 0.0024145817307692323,
                        "current": 0.001494,
                        "maximum": 0.061351,
                        "minimum": 0,
                        "standardDeviation": 0.0029017970782575734
                    },
                    "counts": {
                        "success": 1248,
                        "fail": 0
                    }
                },
                "639ec377a30542cf": {
                    "latency": {
                        "average": 0.0026389089456869013,
                        "current": 0.001241,
                        "maximum": 0.233578,
                        "minimum": 0,
                        "standardDeviation": 0.00695758066274549
                    },
                    "counts": {
                        "success": 1252,
                        "fail": 0
                    }
                }
            },
            "leader": "d3cf079af51fa9a8"
        }
    },
    "event": {
        "dataset": "etcd.leader",
        "duration": 115000,
        "module": "etcd"
    },
    "metricset": {
        "name": "leader"
    },
    "service": {
        "address": "127.0.0.1:2379",
        "type": "etcd"
    }
}

[ruflin]

@sayden For your questions around polishing: I think the answer is yes. Should we open a separate issue to discuss these improvements?

[sayden]

Yes, let's merge this here and open a new issue for discussion.