Remove obsolete TODO for addLazyEvent

[arminru]

See open-telemetry/opentelemetry-specification#840.

[codecov]

Codecov Report

Merging #168 into master will increase coverage by 0.41%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #168      +/-   ##
============================================
+ Coverage     90.52%   90.93%   +0.41%     
  Complexity      285      285              
============================================
  Files            35       35              
  Lines           665      662       -3     
============================================
  Hits            602      602              
+ Misses           63       60       -3     

Impacted Files	Coverage Δ	Complexity Δ
sdk/Metrics/ValueRecorder.php	87.50% <0.00%> (+3.50%)	10.00% <0.00%> (ø%)
sdk/Metrics/UpDownCounter.php	63.63% <0.00%> (+5.30%)	5.00% <0.00%> (ø%)
sdk/Metrics/Counter.php	83.33% <0.00%> (+6.41%)	5.00% <0.00%> (ø%)

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