Introduce new unauthenticated endpoint /metric (#5723) (#5829)

/metric exposes Promethus compatible data for scraping metrics

Fixes: #5723

docs/metric/README.md

@@ -0,0 +1,38 @@
+## Minio Prometheus Metric
+
+Minio server exposes an endpoint for Promethueus to scrape server data at `/minio/metric`.
+
+### Prometheus probe
+Prometheus is used to monitor Minio server information like http request, disk storage, network stats etc.. It uses a config file named `prometheus.yaml` to scrape data from server. The value for `metrics_path` and `targets` need to be configured in the config yaml to specify the endpoint and url as shown:
+```
+scrape_configs:
+  # The job name is added as a label `job=<job_name>` to any timeseries scraped from this config.
+  - job_name: minio
+    metrics_path: /minio/prometheus/metrics
+
+    # metrics_path defaults to '/metrics'
+    # scheme defaults to 'http'.
+
+    static_configs:
+      - targets: ['localhost:9000']
+```
+ Prometheus can be run by executing :
+```
+./prometheus --config.file=prometheus.yml
+```
+
+### List of Minio metric exposed
+Minio exposes the following list of metric to Prometheus
+```
+minio_disk_storage_bytes
+minio_disk_storage_free_bytes
+minio_http_requests_duration_seconds_bucket
+minio_http_requests_duration_seconds_count
+minio_http_requests_duration_seconds_sum
+minio_http_requests_total
+minio_network_received_bytes
+minio_network_sent_bytes
+minio_offline_disks_total
+minio_online_disks_total
+minio_server_uptime_seconds
+```