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 
+For 1st part, process-reboot-cause copies the previous-reboot-cause.txt 
+to "/host/reboot-cause/previous-reboot-cause/" with adding timestamp 
+at the end of file name after processing the reboot-cause.
+Currently previous-reboot-cause.txt is plan text format but this file content will be formatized to be parsed easily.
+Read each reboot-cause information from saved previous-reboot-cause files
+And update reboot-cause information upto 10 entries to state-DB.  
+
+The example shows the previous reboot-cause files stored in /host/reboot-cause/previous-reboot-cause/.
+```
+$ls /host/reboot-cause/previous-reboot-cause/
+previous-reboot-cause-20200903T232033.txt
+previous-reboot-cause-20200902T101105.txt
+previous-reboot-cause-20200902T015048.txt
+```
+The following example shows the content of the previous reboot-cause file - previous-reboot-cause-20200903T232033.txt.
+```
+TIMESTATEMP: "20200903T232033"
+CAUSE: "reboot"
+USER: "admin"
+TIME: "Thu 03 Sep 2020 11:15:30 PM UTC"
+COMMENT: "User issued 'reboot' command [User: admin, Time: Thu 03 Sep 2020 11:15:30 PM UTC]" 
+```
+
+Details of CLI and state-DB given below. 
+Cli command to retrieve the Reboot-cause information
+```
+$ show reboot-cause
+```
+state-DB schema to store the Reboot-cause information
+```
+; Defines information for reboot-cause
+key                     = REBOOT_CAUSE|timestamp         ; last reboot-cause processing time
+; field                 = value
+cause                   = STRING                         ; last reboot cause
+time                    = STRING                         ; time when the last reboot was initiated
+user                    = STRING                         ; user who the last reboot initiated
+comment                 = STRING                         ; unstructured json format data
+```
+Along with data new entry for timestamp will be added up to 10 entries in state_db:  
+
+```
+REBOOT_CAUSE|timestamp
+```
+
+##### Part 2
+Verify that from state-DB data is available via telemetry agent
+
+##### 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. 
+With new design, the reboot-cause will be read from state-DB and displayed with new format.
+`show reboot-cause history` will be added and displays the previous `reboot-cause` upto 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 inside state-DB as follows for the reboot-cause information
+```
+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 11:20:33 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]"  
+```