feat: Add ability to Explain the response plan.

[shahzadlone]

Related Issues

Resolves #325,
Resolved #477 (as CommitSelectNode for now doesn't have any explainable attribute)
Closes #361, Closes #362 and also Closes #363 (Closing the TEXT structured ones because we have decided to only go with JSON approach to avoid the wonky looking client side formatting).

Fixes #486 in commit d940f7b2e6ee54d0ce8b4bd8f5330db60307630f

Description

This PR lays all the ground work needed to have the opt-ability for planner nodes to subscribe to the explain interface if they would like to be exposed to the explaining of the plan graph.

Even though top-level plan node name will be explained after this PR for all explainable nodes, this PR doesn't implement all the explain attributes for all explainable nodes. Here are the planNodes whose attributes can be explained after this PR:

1. selectNode (attributes completed)
2. selectTopNode (attributes completed / has no attributes)
3. createNode (attributes completed)
4. deleteNode (attributes completed)
5. scanNode (some attributes completed)

Request:

query @explain {
 user {
   _key
   age
   name
 }
}

Response:

{
 "data": [
   {
     "explain": {
       "selectTopNode": {
         "selectNode": {
           "filter": null,
           "scanNode": {
             "collectionID": "1",
             "collectionName": "user",
             "filter": null
           }
         }
       }
     }
   }
 ]
}

The other attributes that need to be added are now tracked as sub-tasks / check-list items to #35. They are going to be their own PRs to keep this PR consumable and in order to also avoid having conflicts:

• Explain the attributes of scanNode #474
• Explain the attributes of typeIndexJoin. #475
• Explain the attributes of averageNode. #476
• Explain the attributes of commitSelectNode. #477
• Explain the attributes of countNode. #478
• Explain the attributes of dagScanNode. #479
• Explain the attributes of renderNode. #480
• Explain the attributes of sortNode. #481
• Explain the attributes of sumNode. #482
• Explain the attributes of updateNode. #483

[jsimnz]

Few thoughts, some just suggestions or comments. One notable structural change on the planNode interface.

[AndrewSisley]

Partial review - will continue later

[jsimnz]

Looks great! Everying has mostly been taken care of.

Omitting my comments about the multiscanNode stuff since we discussed that on discord. (TLDR: multiscanNode embedding *scanNode caused issue :/)

Only notable issue is the assumed allocation of map[string]interface{} in Explain() methods. Basically, if a node is explainable, but doens't care to emit attributes, it should be fine to return a nil map. But currently, that will crash the DB since it assumes it returns a non-nil map.

[jsimnz]

LGTM