[agent] Use grpc.NewClient

[yurishkuro]

Which problem is this PR solving?

• Part of Migrate from grpc.Dial to grpc.NewClient #5330

Description of the changes

• Use grpc.NewClient
• Remove irrelevant tests for connection status

How was this change tested?

• CI

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.20%. Comparing base (e5dd759) to head (446d206).

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5392       +/-   ##
===========================================
+ Coverage   55.51%   95.20%   +39.69%     
===========================================
  Files         171      346      +175     
  Lines        9048    16908     +7860     
===========================================
+ Hits         5023    16098    +11075     
+ Misses       3554      610     -2944     
+ Partials      471      200      -271     

Flag	Coverage Δ
badger_v1	10.47% <ø> (ø)
badger_v2	6.42% <ø> (?)
cassandra-3.x	18.37% <ø> (?)
cassandra-4.x	18.37% <ø> (ø)
elasticsearch-5.x	20.91% <ø> (?)
elasticsearch-6.x	20.91% <ø> (?)
elasticsearch-7.x	20.96% <ø> (?)
elasticsearch-8.x	21.14% <ø> (?)
grpc_v1	12.62% <ø> (ø)
grpc_v2	11.54% <ø> (ø)
kafka	10.18% <ø> (ø)
opensearch-1.x	21.00% <ø> (+0.01%)	⬆️
opensearch-2.x	20.98% <ø> (-0.06%)	⬇️
unittests	91.54% <100.00%> (?)

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[jkowall]

LGTM!