Cleanup usage of TimestampUnixNanos and its API

[bogdandrutu]

Updates #2488

Important Changes:

• Rename pdata.TimestampUnixNanos to pdata.Timestamp
• Remove pdata.TimestampUnixNanos and helpers, move them to the pdata.Timestamp type.
• Fix bug around IsZero, this function should return true if the time is January 1, year 1, 00:00:00 UTC not epoch.

[codecov]

Codecov Report

Merging #2549 (e9a4c2a) into main (7d7ae2e) will decrease coverage by 0.00%.
The diff coverage is 98.86%.

@@            Coverage Diff             @@
##             main    #2549      +/-   ##
==========================================
- Coverage   91.69%   91.68%   -0.01%     
==========================================
  Files         266      266              
  Lines       15111    15101      -10     
==========================================
- Hits        13856    13846      -10     
  Misses        871      871              
  Partials      384      384              

Impacted Files	Coverage Δ
consumer/pdata/common.go	98.91% <ø> (-0.01%)	⬇️
...r/internal/scraper/cpuscraper/cpu_scraper_linux.go	100.00% <ø> (ø)
...l/scraper/diskscraper/disk_scraper_others_linux.go	100.00% <ø> (ø)
...raper/filesystemscraper/filesystem_scraper_unix.go	100.00% <ø> (ø)
...rnal/scraper/memoryscraper/memory_scraper_linux.go	100.00% <ø> (ø)
...craper/processesscraper/processes_scraper_linux.go	100.00% <ø> (ø)
...al/scraper/processscraper/process_scraper_linux.go	100.00% <ø> (ø)
translator/internaldata/traces_to_oc.go	86.79% <75.00%> (ø)
consumer/pdata/generated_log.go	100.00% <100.00%> (ø)
consumer/pdata/generated_metrics.go	100.00% <100.00%> (ø)
... and 26 more

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[tigrannajaryan]

Looks good, except maybe change of behavior for zero time. We had a bug which was fixed by this. I don't remember what exactly, maybe try digging using git blame to see when was it introduced.

[bogdandrutu]

Even if we fixed other bug, we did that by adding a new bug because IsZero on the time package means year 1 not 1970

[bogdandrutu]

@tigrannajaryan I think it was a simple bug added by myself #1550