[ISSUE #8454] Active brokers number should be initailized to 1 in broker heartbeat manager.

[ShannonDing]

Which Issue(s) This PR Fixes

Fixes #8454

Brief Description

active brokers number should be initailized to 1 in broker heartbeat manager.

How Did You Test This Change?

wait to add test cases for this two class.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 45.88%. Comparing base (304987d) to head (cd489fa).

Files	Patch %	Lines
.../impl/heartbeat/DefaultBrokerHeartbeatManager.java	0.00%	1 Missing ⚠️
...ler/impl/heartbeat/RaftBrokerHeartBeatManager.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #8453      +/-   ##
=============================================
- Coverage      45.92%   45.88%   -0.04%     
+ Complexity     11104    11098       -6     
=============================================
  Files           1274     1274              
  Lines          89001    89001              
  Branches       11441    11441              
=============================================
- Hits           40875    40841      -34     
- Misses         43027    43063      +36     
+ Partials        5099     5097       -2     

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