sdk/log/logtest: Add RecordFactory

[pellared]

Fixes #5192.

Follows @MrAlias idea to make the implementation simpler than #5234

See test example for usage.

The benefit of NOT using a fluent builder is that all code that uses RecordFactory would look similar.

When using a fluent builder one could write:

b := logtest.RecordBuilder{}
b = b.SetInstrumentationScope(instrumentation.Scope{Name: "myapp"})
b = b.SetBody(logapi.StringValue("foo"))
r := b.Record()

and someone else could write:

r := logtest.RecordBuilder{}.
  SetInstrumentationScope(instrumentation.Scope{Name: "myapp"}).
  SetBody(logapi.StringValue("foo")).
  Record()

Moreover, fluent builder also makes it easy to make a mistake in test code. E.g. for current implementation the code below would return an empty record (sic!):

b := logtest.RecordBuilder{}
b.SetInstrumentationScope(instrumentation.Scope{Name: "myapp"})
b.SetBody(logapi.StringValue("foo"))
r := b.Record()

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 84.6%. Comparing base (9656d0a) to head (59f0956).

❗ Current head 59f0956 differs from pull request most recent head fff83e7. Consider uploading reports for the commit fff83e7 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5258     +/-   ##
=======================================
- Coverage   84.6%   84.6%   -0.1%     
=======================================
  Files        259     259             
  Lines      17384   17388      +4     
=======================================
  Hits       14711   14711             
- Misses      2362    2366      +4     
  Partials     311     311             

Files	Coverage Δ
sdk/log/logtest/factory.go	84.6% <84.6%> (ø)

... and 3 files with indirect coverage changes

[XSAM]

LGTM