Add default metrics supported by OpenCensus system metrics package

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.).