[ISSUE #91] remove lastupdateTimestamp for offset metric

[cserwen]

What is the purpose of the change

Fix #91

Brief changelog

lastUpdateTimestamp should not be put into lables.

Verifying this change

XXXX

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[codecov-commenter]

Codecov Report

Merging #102 (4269107) into master (8bc34bc) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff            @@
##             master    #102   +/-   ##
========================================
  Coverage      0.35%   0.35%           
  Complexity        2       2           
========================================
  Files            30      30           
  Lines          2242    2240    -2     
  Branches        195     195           
========================================
  Hits              8       8           
+ Misses         2234    2232    -2     

Impacted Files	Coverage Δ
...cketmq/exporter/collector/RMQMetricsCollector.java	0.00% <0.00%> (ø)

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[ShannonDing]

LGTM