Add design/discussion doc auto-pruning

enhancements/auto-prune-design-discussion.md

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+# Advance auto-pruning 
+- ## Actual advance auto-pruneing doc [here](https://docs.google.com/document/d/1259TGcQxVrxycrFS1YbqVUyebpc7cg8_qpogtdUpMPg/edit)
+
+- ## Initial auto-pruning doc [here](https://docs.google.com/document/d/1DrIzaN5jm5QV_TysArJO0W4Eurfh5z_GR-eaxkGuz88/edit)
+
+## Acceptance: 
+Admin can configure the operator to automatically prune the pipelinerun with the following configurations:
+
+- Keep last N pipelineruns for each pipeline
+- Keep pipelineruns younger than D days for each pipeline
+- Keep pipelineruns and taskruns that are younger than D days
+- Support per namespace auto-prune
+
+    - Do we want to to prune all the ns by default andor sikp pruning for annotated ones
+    - Or Do we want to to disable prune for all the ns by default and enable pruning for annotated ones
+- Do not delete pipelineruns/taskruns which are in running state.
+- Should be enabled by default 
+
+
+## How are we gonna tackle these
+- Keep last N pipelineruns for each pipeline
+
+        Here tkn can help out 
+        tkn taskrun delete --task=abcTask --keep=2 -n foo 
+        tkn pipelinerun delete --pipeline=abcPipeline --keep=2 -n foo
+    - Question : 
+        - How to get the abcPipeline/abcTask from user
+
+
+- Keep pipelineruns/taskruns younger than D days for each pipeline/task
+
+        Here also tkn will do it.
+        tkn taskrun delete --task=abcTask --keep-since=<n-minutes> -n foo 
+        tkn pipelinerun delete --pipeline=abcPipeline --keep-since=<n-minutes> -n foo
+        The problem is we already have keep now we want to have keep-since, 
+        how do we take input from the user.
+
+
+
+- Keep pipelineruns and taskruns that are younger than D days
+
+        Here also tkn will do it.
+        tkn taskrun delete --keep-since=<n-minutes> -n foo 
+        tkn pipelinerun delete --keep-since=<n-minutes> -n foo
+        We may need to address keep and keep-since conflict
+            In my opinion this could be mutually exclusive.
+- Support per namespace auto-prune
+
+        List all the namespaces which have specific annotation. As suggested in the issue>
+            Annotation:
+                pipeline.openshift.io/namespace/prunable (open for other options)
+                pipeline.openshift.io/prunable
+            Changed to operator.tekton.dev
+
+            Question: Do we assume that user will add these annotations to the namespaces.
+- Do not delete pipelineruns/taskruns which are in running state. (this came from a pr discussion)
+
+        - We do have keep and keep-since, with keep-since we specify time, so 
+          the user will already know that something is running.
+        - But if we want to get this in , would it make sense to add this in 
+          cli a flag which ignore all the pipelinerun/taskrun running 
+
+
+
+
+
+
+
+### Tasks 
+- Keep last N pipelineruns for each pipeline				- **Done** 
+- Keep pipelineruns younger than D days for each pipeline		- **Done**
+- Keep pipelineruns and taskruns that are younger than D days		- **Done**
+- Support per namespace auto-prune
+- Annotate ns with skip:true/false				-   **Done** 
+- Create a separate cronjob if a diff schedule value is set in the ns annotation pipeline.openshift.io/prune.schedule”	-	**Done**
+- If the annotation is deleted on ns or if skip:true is added and if there is an existing cron we should  delete it.	-	**Done**
+- Do not delete pipelineruns/taskruns which are in running state.
+
+    - Need cli release for taskrun, pipelinerun 
+    - Should be enabled by default 
+- Added to all, basic, lite of kubernetes and openshift. **Done** 
+- CLI in review
+    - https://github.com/tektoncd/cli/pull/1435/files   **Done**
+    - https://github.com/tektoncd/cli/pull/1436/files    **InReview**
+    - Created an issue https://github.com/tektoncd/cli/issues/1442
+
+## Question:
+    Q1 Annotation are assumed to be done by ClusterAdmin/user with some privilege  
+	Its fine to assume.
+
+    Q2 Is pruning done by default
+	Its should not be done if there is an update.
+	Its should be enabled by default, if new install
+
+    Q3 Keep-since should be ignored
+	Give doc about it, any one of the item should be specified. 
+
+project-execute-extended-command
+Open questions
+Relation with the Results and it’s availability/support in OpenShift Pipelines (and devconsole, …) ?
+
+## Updates on 23rd Sep
+This comes after pr review for the advance prune features like pruning per namespaces, and pruning with related resource https://github.com/tektoncd/cli/pull/1445
+
+We figured out the approach we are taking, which is to  put annotations on the namespaces, is not scalable.
+"operator.tekton.dev/prune.resources.pipelineNames"
+"operator.tekton.dev/prune.resources.taskNames"
+The names specified in the above annotation were used as related resource names. Like delete all the pipelinerun related to pipelineNames ( with other configs like keep, keepsince)
+
+
+## Another Approach Suggested
+
+    We remove  the earlier defined annotation
+    "operator.tekton.dev/prune.resources.pipelineNames"
+    "operator.tekton.dev/prune.resources.taskNames"
+    And introduce a new  annotation "operator.tekton.dev/prune.foreach-pipeline"
+
+    If this annotation is found we get all the pipeline in the namespace and apply other configs like keep, and keepsince while deletion.
+    This comes out as , Delete pipelineruns related to pipeline keep/keepsicne for all the pipelines in that namespace.
+
+    With this instead of take specific pipeline names from users we just ask users if they want to keep x pipelineruns for each pipeline in the namespace.
+
+