Add reboot-cause information to telemetry

doc/system-telemetry/reboot-cause.md

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+# Reboot-cause information via telemetry agent
+
+## Revision
+
+| Rev | Date     | Author      | Change Description |
+|:---:|:--------:|:-----------:|--------------------|
+| 0.1 | 09/02/20 | Sujin Kang  | Initial version    |
+
+## Scope
+-Enable sonic streaming telemetry agent to send Reboot-cause information
+
+### Enable sonic streaming telemetry agent to send Reboot-cause information
+
+#### Part 1
+During the boot, the process-reboot-cause processes the last reboot-cause based on the hardware reboot-cause
+and the software reboot-cause information and creates previous-reboot-cause.txt with the information as it does currently.
+In addition to save the history of the previous reboot-cause, `process-reboot-cause` will save the previous
+reboot cause information to "/host/reboot-cause/previous-reboot-cause/" with adding timestamp at the end of file name.
+And the file will be formatted to be parsed easily.
+
+The example shows the previous reboot-cause files stored in /host/reboot-cause/previous-reboot-cause/.
+```
+$ls /host/reboot-cause/previous-reboot-cause/
+previous-reboot-cause-20200903T232033.txt
+previous-reboot-cause-20200902T101105.txt
+previous-reboot-cause-20200902T015048.txt
+...
+```
+The following example shows the content of the previous reboot-cause file - previous-reboot-cause-20200903T232033.txt.
+```
+[
+    {
+        gen_time: "20200903T232033",
+        cause: "reboot",
+        user: "admin",
+        time: "Thu 03 Sep 2020 11:15:30 PM UTC",
+        comment: ""
+    }
+]
+```
+
+#### Part 2
+A new service named as `post-process-reboot-cause.service` which will retrieve the saved reboot-cause files and read each reboot-cause information from the files
+and save the reboot-cause information up to 10 entries to state-DB.
+Verify the information from state-DB data is available via the cli command `show reboot-cause history` which is extended from `show reboot-cause`.
+
+##### Reboot Cause Schema in state-DB
+
+Here is the definition of Reboot-cause schema which will be stored in state-DB.
+```
+; Defines information for reboot-cause
+key                     = REBOOT_CAUSE|<timestamp>         ; last reboot-cause processing time
+; field                 = value
+cause                   = STRING                         ; last reboot causek
+time                    = STRING                         ; time when the last reboot was initiated
+user                    = STRING                         ; user who the last reboot initiated
+comment                 = STRING                         ; unstructured json format data
+```
+
+##### CLI output  and corresponding structure in state-DB for reboot-cause information
+
+###### reboot-cause information
+
+Currently `show reboot-cause` displays the last reboot-cause and performing `cat /host/reboot-cause/previous-reboot-cause.txt` to show the reboot-cause.
+This will be same as current design.
+With new design, `show reboot-cause history` will be added to display the previous `reboot-cause` up to 10 entries from state-DB.
+
+The example shows the output of `show reboot-cause` which is same as current output and displays only the last reboot-cause.
+```
+$ show reboot-cause
+User issued 'reboot' command [User: admin, Time: Thu 03 Sep 2020 11:15:30 PM UTC]
+```
+Above output will be stored in the previous-reboot-cause.txt file and the reboot-cause information is also stored in state-DB as follows.
+```
+REBOOT_CAUSE|20200903T112033
+"cause"
+"reboot"
+"time"
+"Thu 03 Sep 2020 11:15:30 PM UTC"
+"user"
+"admin"
+"comment"
+"User issued 'reboot' command [User: admin, Time: Thu 03 Sep 2020 11:15:30 PM UTC]"
+```
+
+The example shows the output of `show reboot-cause history` and the previous reboot cause stored in state-DB in addition to the last reboot-cause.
+```
+$ show reboot-cause history
+TIMESTAMP       REBOOT-CAUSE        Details
+20200903T232033 reboot              User issued 'reboot' command [User: admin, Time: Thu 03 Sep 2020 11:15:30 PM UTC]
+20200902T101105 Unknown             Unknown
+20200902T015048 fast-reboot         User issued 'fast-reboot' command [User: admin, Time: Wed 02 Sep 2020 01:48:33 AM UTC]
+```
+Above output will be stored inside state-DB as follows for the previous reboot-cause in addition to the last reboot-cause
+```
+REBOOT_CAUSE|20200902T101105
+"cause"
+"Unknown"
+"time"
+""
+"user"
+""
+"comment"
+"Unknown"
+```
+```
+REBOOT_CAUSE|20200902T015048
+"cause"
+"fast-reboot"
+"time"
+"Wed 02 Sep 2020 01:48:33 AM UTC"
+"user"
+"admin"
+"comment"
+"User issued 'fast-reboot' command [User: admin, Time: Wed 02 Sep 2020 01:48:33 AM UTC]"
+```