Add ResourceMetrics method

[cesnietor]

Similar to #218, this adds the Resource method equivalent to /minio/v2/metrics/resource endpoint.

Also renames a method to be more clear on what it does.

Test Steps:

• Create a client
• Run Metrics API
• Parse the response to string to visualize it

ctx := context.Background()
madminClient, _ := madmin.NewMetricsClient(
		"http://localhost:9000",
		"accesskey",
		"secretKey",
		false,
	)
metrics, _ := metricsClient.ResourceMetrics(ctx)

result, _ := json.MarshalIndent(metrics, "", " ")
log.Println(string(result))

Output would look like:

[
 {
  "name": "someMetrics",
  "help": "Some help",
  "type": "GAUGE",
  "metrics": [
   {
    "value": "1.76933784e+08"
   }
  ]
 },
 {
  "name": "someMetrics",
  "help": "Some other help.",
  "type": "COUNTER",
  "metrics": [
   {
    "value": "2.59015547e+08"
   }
  ]
 },
 ...
 ]

[anjalshireesh]

LGTM. One suggestion to reduce code duplication.

[anjalshireesh]

The code for each of these metrics functions seems almost same, except for the relativePath. It can be put into a common function, something like:

// FetchMetrics - returns Metrics of given subsystem in Prometheus format
func (client *MetricsClient) FetchMetrics(ctx context.Context, subSys string) ([]*prom2json.Family, error) {
	reqData := metricsRequestData{
		relativePath: "/v2/metrics/" + subSys,
	}

	// Execute GET on /minio/v2/metrics/<subSys>
	resp, err := client.executeGetRequest(ctx, reqData)
	if err != nil {
		return nil, err
	}
	defer closeResponse(resp)

	if resp.StatusCode != http.StatusOK {
		return nil, httpRespToErrorResponse(resp)
	}

	return parsePrometheusResults(io.LimitReader(resp.Body, metricsRespBodyLimit))
}

[cesnietor]

@anjalshireesh I like it, I've done it, thanks.