chore: fixing issue when metric time is 0 in document-load plugin

[obecny]

Which problem is this PR solving?

• During working on web collector I discovered a bug with document load and when metric is 0 (for example "secureConnectionStart"). The plugin was treated this metric as available and it was shown as metric with negative value

error image

[mayurkale22]

LGTM

[codecov-io]

Codecov Report

Merging #546 into master will decrease coverage by 2.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #546      +/-   ##
==========================================
- Coverage   92.28%   90.24%   -2.05%     
==========================================
  Files         151      149       -2     
  Lines        7337     7449     +112     
  Branches      647      662      +15     
==========================================
- Hits         6771     6722      -49     
- Misses        566      727     +161

Impacted Files	Coverage Δ
...telemetry-plugin-document-load/src/documentLoad.ts	98.13% <100%> (+0.03%)	⬆️
...pentelemetry-core/test/internal/validators.test.ts	50% <0%> (-50%)	⬇️
...elemetry-core/test/trace/spancontext-utils.test.ts	55.55% <0%> (-44.45%)	⬇️
...lemetry-core/test/trace/ProbabilitySampler.test.ts	56.52% <0%> (-43.48%)	⬇️
...s/opentelemetry-core/test/trace/NoopTracer.test.ts	60% <0%> (-40%)	⬇️
...s/opentelemetry-core/test/context/B3Format.test.ts	63.39% <0%> (-36.61%)	⬇️
...ges/opentelemetry-core/test/trace/NoopSpan.test.ts	63.63% <0%> (-36.37%)	⬇️
...s/opentelemetry-core/test/trace/tracestate.test.ts	65.06% <0%> (-34.94%)	⬇️
...ntelemetry-core/test/trace/NoRecordingSpan.test.ts	71.42% <0%> (-28.58%)	⬇️
...ackages/opentelemetry-core/src/platform/node/id.ts	71.42% <0%> (-28.58%)	⬇️
... and 24 more

[mayurkale22]

/cc @draffensperger