Add controller API to get allLiveInstances

[tibrewalpratik17]

label:
api

Adding a controller API to get all live instances in the cluster.

• We fetch all instances for a cluster for attribution internally. Some of them are dead and causes issues in the attribution logic. Having /liveinstances endpoint will help in having the correct set of valid instances.

Updated UTs to test the change.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 61.71%. Comparing base (59551e4) to head (3a1a09e).
Report is 21 commits behind head on master.

Files	Patch %	Lines
...er/api/resources/PinotInstanceRestletResource.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #12498      +/-   ##
============================================
- Coverage     61.75%   61.71%   -0.05%     
  Complexity      207      207              
============================================
  Files          2436     2449      +13     
  Lines        133233   133505     +272     
  Branches      20636    20686      +50     
============================================
+ Hits          82274    82386     +112     
- Misses        44911    45037     +126     
- Partials       6048     6082      +34     

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[ankitsultana]

Would this also list instances that have temporarily disconnected from Helix? (due to GC, regular restart, etc.)

If not, might be good to call it out explicitly in the description.

[tibrewalpratik17]

I tested via the following:

• Restarted a node in one of our clusters. During the restart we did not get the instance id of that node in the /liveinstances ZK path. Post restart, we got the instance back.

[ankitsultana]

change lgtm. tagged @Jackie-Jiang to see if he has any opinions on this. I see that there was a controller task updated in #10027 to fix this issue. But having this endpoint might anyways be useful.

[satishd]

Thanks @tibrewalpratik17 for the PR. The added API /liveinstances seems reasonable to me.

[Jackie-Jiang]

LGTM otherwise