otlpmetric: Fix serialization of time.Time zero values

[pellared]

Fixes #4268

Remarks:
I hope that adding a test case for one field is good enough (and that I do not need to add test cases for all possible fields).

[codecov]

Codecov Report

Merging #4271 (03df5e7) into main (4570292) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4271     +/-   ##
=======================================
- Coverage   83.6%   83.5%   -0.1%     
=======================================
  Files        181     181             
  Lines      14184   14189      +5     
=======================================
+ Hits       11860   11861      +1     
- Misses      2096    2100      +4     
  Partials     228     228             

Impacted Files	Coverage Δ
...s/otlp/otlpmetric/internal/transform/metricdata.go	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

[pellared]

CC @anil181