sdk/metric: Set StartTime for gauge metric points

[pellared]

Fixes #4269

Remarks

We may postpone until open-telemetry/opentelemetry-specification#3540 (or open-telemetry/opentelemetry-specification#3576) clarifies StartTimeUnixNano.

On the other hand, I am not sure when it happens. During the spec spec SIG meeting it looks that there is no common agreement what the value should be. However, people see that it is worth to set it as a "non-zero" value. I decided it may be worth to make a baby step. This is why I decided propose this PR. Especially that I think it is inline with the current recommendation:

(optional) A timestamp (start_time_unix_nano) which best represents the first possible moment a measurement could be recorded. This is commonly set to the timestamp when a metric collection system started.

[codecov]

Codecov Report

Merging #4345 (da73fc9) into main (e08359f) will increase coverage by 0.0%.
The diff coverage is 100.0%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4345   +/-   ##
=====================================
  Coverage   83.4%   83.4%           
=====================================
  Files        185     185           
  Lines      14332   14338    +6     
=====================================
+ Hits       11958   11966    +8     
+ Misses      2147    2145    -2     
  Partials     227     227           

Impacted Files	Coverage Δ
sdk/metric/internal/aggregate/aggregate.go	100.0% <100.0%> (ø)
sdk/metric/internal/aggregate/lastvalue.go	100.0% <100.0%> (ø)
sdk/metric/pipeline.go	91.2% <100.0%> (+<0.1%)	⬆️

... and 1 file with indirect coverage changes

[pellared]

Closing per #4269 (comment)