Add default metrics supported by OpenCensus system metrics package

Author: mayurkale22

metrics/SystemMetrics.md

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+# System Metrics
+This document serves to document the "look and feel" of the OpenCensus API for auto-collection of system metrics such as processes, memory & CPU usage, disks, etc.
+
+## Supported platforms
+TODO: Add list of supported platforms.
+
+## Common metrics collection
+These are some default metrics supported by OpenCensus system metrics package/contrib. This is to ensure consistency across language implementations, as far as practical.
+
+| Metric Name                               | Unit  | Type                       | Description                                                                                                | Labels                 |
+|-------------------------------------------|-------|----------------------------|-----------------------------------------------------------------------------------------------|-----------------|
+| system/cpu_seconds/total                  | s     | Int64 Cumulative           | Total kernel/system user CPU in seconds.                                                          | Hostname, Mode          |
+| system/processes/created                  | 1     | Int64 Cumulative           | Total number of times a process was created.                                                      | Hostname                |
+| system/processes/running                  | 1     | Int64 Gauge                | Total number of running processes.                                                                | Hostname                |
+| system/disk/total                         | By    | Int64 Gauge                | The total available system disk space in bytes.                                                   | Hostname, Volume        |
+| system/disk/used                          | By    | Int64 Gauge                | The total used system disk space in bytes.                                                        | Hostname, Volume        |
+| process/heap/total                        | By    | Int64 Gauge                | The process's total allocated heap size in bytes.                                                 | Hostname                |
+| process/heap/used                         | By    | Int64 Gauge                | The process's total used heap size in bytes.                                                      | Hostname                |
+| process/memory/rss                        | By    | Int64 Gauge                | Resident memory size in bytes.                                                                    | Hostname                |
+| process/file_descriptor/max               | 1     | Int64 Gauge                | The maximum limit of file descriptor count.                                                       | Hostname                |
+| process/file_descriptor/open              | 1     | Int64 Gauge                | The number of open file descriptor count.                                                         | Hostname                |
+| process/start_time                        | s     | Int64 Cumulative           | The start time of the process since unix epoch in seconds.                                        | Hostname                |
+| process/cpu/usage                         | s     | Int64 Cumulative           | The total user and system CPU time spent in seconds.                                              | Hostname                |
+
+### Labels
+The label keys associated with the above metrics.
+
+| Label Key          | Description                                                                                              |
+|--------------------|----------------------------------------------------------------------------------------------------------|
+| Mode               | The amount of time spent by the CPU in state (i.e. `user, nice, system, idle, iowait`).                  |
+| Volume             | The path the fs is mounted on (i.e. `/, /data`)                                                          |
+| Hostname           | The hostname of the host (i.e. `opencensus-test`).                                                       |
+
+## Language specific metrics collection
+TODO: Add language-specific documentation for the list of supported metrics collection (e.g. garbage collection, threads, classloaders, etc.).