fix: use system clock for timestamps and perf clock for durations

[dyladan]

Fixes #852

Because the timestamps generated from the performance timer cannot be trusted, we can only use them to generate durations, not real timestamps. This PR introduces the following behavior:

User provides start AND end timestamps

• Start Time from user
• Duration is calculated from input times
• End time is calculated from start time input and duration

User provides start timestamp

• Start time from user
• Duration calculated from input start time and system clock
• End time is calculated from start time input and duration

User provides end timestamp

• Start time from system clock
• Duration calculated from system clock start time and input end time
• End time is calculated from system clock start time and duration

User provides no timestamps

• Start time from system clock
• Duration is calculated using the performance clock
• End time is calculated using system clock start time and duration

[obecny]

unit tests ?

[codecov-io]

Codecov Report

Merging #1019 into master will decrease coverage by 0.03%.
The diff coverage is 92.18%.

@@            Coverage Diff             @@
##           master    #1019      +/-   ##
==========================================
- Coverage   95.00%   94.96%   -0.04%     
==========================================
  Files         212      212              
  Lines        8806     8862      +56     
  Branches      796      802       +6     
==========================================
+ Hits         8366     8416      +50     
- Misses        440      446       +6     

Impacted Files	Coverage Δ
packages/opentelemetry-core/src/common/time.ts	88.73% <16.66%> (-6.66%)	⬇️
packages/opentelemetry-tracing/src/Span.ts	98.92% <100.00%> (-1.08%)	⬇️
...telemetry-tracing/test/BasicTracerProvider.test.ts	100.00% <100.00%> (ø)

[vmarchaud]

i believe we should check startTime for undefined instead of null

[Flarna]

Mind the missing second = - startTime != null is the same as (startTime !== null) || (startTime !== undefined).

[vmarchaud]

Didn't see that indeed, i would prefer to have strict check so its easier to read the code though

[Flarna]

My feeling is that this is quite a heavy change for a bug which was fixed in NodeJs 8.12.0.
For browsers this may be different as they run on machins getting hibernated frequently.
Mixing different time sources tends to be problematic.
What about making the time source to use configurable? I could even think about allow users to provide their time provider.

[pichlermarc]

Looks like this was replaced by #3129, which was replaced by #3134, should we close this @dyladan? Looks like stale bot never gets so far. 🙂