API: support metrics for prometheus.(#2899)

[chundonglinlin]

Support metrics for Prometheus, Prometheus can call metrics, likely:

http://127.0.0.1:1985/metrics

Prometheus config:

scrape_configs:
    static_configs:
      - targets: ['127.0.0.1:1985']
        labels:
          group: '127.0.0.1:1985'

SRS config:

listen              1935;
max_connections     1000;
daemon              off;
srs_log_tank        console;

exporter {
    # Whether exporter is enabled.
    # Overwrite by env SRS_EXPORTER_ENABLED
    # default: off
    enabled         on;
    # The logging label to category the cluster servers.
    # Overwrite by env SRS_EXPORTER_LABEL
    label cn-beijing;
    # The logging tag to category the cluster servers.
    # Overwrite by env SRS_EXPORTER_TAG
    tag cn-edge;
}

http_api {
    enabled         on;
    listen          1985;
}
vhost __defaultVhost__ {
}