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 `determine-reboot-cause` service ( previously `process-reboot-cause`) determines the last reboot-cause based on the hardware reboot-cause
+and the software reboot-cause information and `determine-reboot-cause` service will save the formatted last previous
+reboot cause information to "/host/reboot-cause/previous-reboot-cause/" with adding timestamp at the end of file name.
+`determine-reboot-cause` also will also create a symbolic link of the last reboot cause file to "/host/reboot-cause/previous-reboot-cause.txt"
+
+The example shows the previous reboot-cause files stored in /host/reboot-cause/previous-reboot-cause/.
+```
+admin@sonic:~$ ls /host/reboot-cause/previous-reboot-cause/
+previous-reboot-cause-2020_10_09_01_56_59.txt
+previous-reboot-cause-2020_10_09_02_00_53.txt
+previous-reboot-cause-2020_10_09_02_33_06.txt
+previous-reboot-cause-2020_10_09_04_53_58.txt
+...
+```
+
+The following example shows the content of the previous reboot-cause file - previous-reboot-cause-2020_10_09_04_53_58.txt.
+```
+admin@sonic:~$ sudo cat /host/reboot-cause/previous-reboot-cause/previous-reboot-cause-2020_10_09_04_53_58.txt
+{"comment": "", "gen_time": "2020_10_09_04_53_58", "cause": "warm-reboot", "user": "admin", "time": "Fri Oct  9 04:51:47 UTC 2020"}
+```
+```
+[
+    {
+        gen_time: "2020_10_09_04_53_58",
+        cause: "warm-reboot",
+        user: "admin",
+        time: "Fri Oct  9 04:51:47 UTC 2020"
+        comment: ""
+    }
+]
+```
+
+#### Part 2
+A new service named as `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
+
+`show reboot-cause` displays the last reboot-cause saved in "/host/reboot-cause/previous-reboot-cause.txt".
+This will be same as current design but the file will be symbolic-linked to the last saved file with time stamp.
+With new design, `show reboot-cause history` will be added to display the previous reboot-cause information 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.
+```
+admin@sonic:~$ show reboot-cause
+User issued 'warm-reboot' command [User: admin, Time: Fri Oct  9 04:51:47 UTC 2020]
+```
+
+And the reboot-cause information is also stored in state-DB as follows.
+```
+REBOOT_CAUSE|2020_10_09_04_53_58
+"cause"
+"warm-reboot"
+"time"
+"Fri Oct  9 04:51:47 UTC 2020"
+"user"
+"admin"
+"comment"
+""
+```
+
+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.
+```
+admin@sonic:~$ show reboot-cause history
+name                 cause        time                          user    comment
+-------------------  -----------  ----------------------------  ------  ---------
+2020_10_09_04_53_58  warm-reboot  Fri Oct  9 04:51:47 UTC 2020  admin
+2020_10_09_02_33_06  reboot       Fri Oct  9 02:29:44 UTC 2020  admin
+2020_10_09_02_00_53  fast-reboot  Fri Oct  9 01:58:04 UTC 2020  admin
+2020_10_09_01_56_59  reboot       Fri Oct  9 01:53:49 UTC 2020  admin
+```
+Above output will be stored inside state-DB as follows for the previous reboot-cause in addition to the last reboot-cause
+```
+admin@sonic:~$ redis-cli -n 6 keys "REBOOT_CAUSE|*"
+1) "REBOOT_CAUSE|2020_10_09_04_53_58"
+2) "REBOOT_CAUSE|2020_10_09_02_33_06"
+3) "REBOOT_CAUSE|2020_10_09_02_00_53"
+4) "REBOOT_CAUSE|2020_10_09_01_56_59"
+
+admin@sonic:~$ redis-cli -n 6 hgetall "REBOOT_CAUSE|2020_10_09_04_53_58"
+1) "cause"
+2) "warm-reboot"
+3) "time"
+4) "Fri Oct  9 04:51:47 UTC 2020"
+5) "user"
+6) "admin"
+7) "comment"
+8) ""
+
+admin@sonic:~$ redis-cli -n 6 hgetall  "REBOOT_CAUSE|2020_10_09_02_33_06"
+1) "cause"
+2) "reboot"
+3) "time"
+4) "Fri Oct  9 02:29:44 UTC 2020"
+5) "user"
+6) "admin"
+7) "comment"
+8) ""
+
+admin@sonic:~$ redis-cli -n 6 hgetall  "REBOOT_CAUSE|2020_10_09_02_00_53"
+1) "cause"
+2) "fast-reboot"
+3) "time"
+4) "Fri Oct  9 01:58:04 UTC 2020"
+5) "user"
+6) "admin"
+7) "comment"
+8) ""
+
+admin@sonic:~$ redis-cli -n 6 hgetall  "REBOOT_CAUSE|2020_10_09_01_56_59"
+1) "cause"
+2) "reboot"
+3) "time"
+4) "Fri Oct  9 01:53:49 UTC 2020"
+5) "user"
+6) "admin"
+7) "comment"
+8) ""
+
+```