Faster ASP.Net Core implementation of Exporter Prometheus

[ysma500]

As a dev using ASP.Net Core MVC, I mainly use the file appsettings.json to assign different settings. We could use that file to do the heavy lifting of setting-up Prometheus Exporter.
Instead of using this method to init the Exporter:

        private TagKey CodeKey;
        private TagKey RouteKey;
        private IMeasureLong ReqMeasure;
        private IStatsRecorder statsRecorder;
        public RequestMetrics(int httpServerPort=8080)
        {
            CodeKey = TagKey.Create("status_code");
            RouteKey = TagKey.Create("route");
            // Metric type: name, description, unit
            ReqMeasure = MeasureLong.Create("measures_requests", "counter of requests", "req");
            IViewName ReqViewName = ViewName.Create("views_requests_total");
            // Counter type, default to 0 at start if wo/params
            IAggregation ReqViewType = Count.Create();
            // Build view: (IViewName name, string description, IMeasure measure, IAggregation aggregation, IReadOnlyList<ITagKey> columns)
            IView RequestView = View.Create(
                ReqViewName,
                " requests made over time",
                ReqMeasure,
                ReqViewType,
                new List<TagKey>() { RouteKey, CodeKey });

            statsRecorder = Stats.StatsRecorder;
            IViewManager viewManager = Stats.ViewManager;
            viewManager.RegisterView(RequestView);
            var exporter = new PrometheusExporter(
            new PrometheusExporterOptions()
            {
                Url = String.Format("http://+:{0}/metrics/", httpServerPort.ToString())
            },
            viewManager);

            // Starting of the prometheus provider (http server)
            exporter.Start();

        }

We could promote something like this:

"PrometheusExporterOptions": {
    "Hostname": "+",
    "ListenOnPort": "8080",
    "Path": "/",
    "AppName": [ "metrics" ],
    "Views": [
      {
        "Name": "views_requests_total",
        "Description": " requests made over time",
        "Measure": {
          "Name": "measures_requests",
          "Description": "counter of requests",
          "Unit": "req",
          "Type": "long"
        },
        "Type": "Count",
        "Tags": [
          {
            "Name": "status_code",
            "Type": "int"
          },
          {
            "Name": "route",
            "Type": "string"
          }
        ]
      }
    ]
  },

If you are interested, I could work on the parsing of the appsettings file. My json file structure might not be optimal, comments are welcomed. The only risk from this implementation is to make sure to not duplicate TagKeys considering they're declared in each of the "Views" themself.

[AlexeiScherbakov]

Something like:
services.Configure(configuration.GetSection("PrometheusExporterOptions"));

then this will be avaliable via IOptions<>

Related issue - #101

[cijothomas]

Closing this issue as the stats based exporter is removed.