[repo] Remove #nullable enable

[Kielek]

Follow up: #2251
Towards #894

Changes

Remove #nullable enable
in addition, it disables nullable per-file in Geneva.tests.

It is done to avoid IDE0240 while migrating to .NET9

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• [ ] Appropriate CHANGELOG.md files updated for non-trivial changes
• [ ] Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.05%. Comparing base (71655ce) to head (efea6c4).
Report is 577 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2290      +/-   ##
==========================================
- Coverage   73.91%   73.05%   -0.87%     
==========================================
  Files         267      359      +92     
  Lines        9615    13737    +4122     
==========================================
+ Hits         7107    10035    +2928     
- Misses       2508     3702    +1194     

Flag	Coverage Δ
unittests-Contrib.Shared.Tests	77.13% <ø> (?)
unittests-Exporter.Geneva	62.37% <ø> (?)
unittests-Exporter.InfluxDB	95.14% <ø> (?)
unittests-Exporter.Instana	74.86% <ø> (?)
unittests-Exporter.OneCollector	94.34% <ø> (?)
unittests-Exporter.Stackdriver	79.26% <ø> (?)
unittests-Extensions	88.63% <ø> (?)
unittests-Extensions.Enrichment	100.00% <ø> (?)
unittests-Instrumentation.AWS	86.27% <ø> (?)
unittests-Instrumentation.AspNet	76.73% <ø> (?)
unittests-Instrumentation.AspNetCore	70.33% <ø> (?)
unittests-Instrumentation.ConfluentKafka	14.37% <ø> (?)
unittests-Instrumentation.ElasticsearchClient	79.87% <ø> (?)
unittests-Instrumentation.EntityFrameworkCore	57.06% <ø> (?)
unittests-Instrumentation.EventCounters	76.36% <ø> (?)
unittests-Instrumentation.GrpcNetClient	79.61% <ø> (?)
unittests-Instrumentation.Hangfire	93.58% <ø> (?)
unittests-Instrumentation.Http	73.57% <ø> (?)
unittests-Instrumentation.Owin	85.97% <ø> (?)
unittests-Instrumentation.Process	100.00% <ø> (?)
unittests-Instrumentation.Quartz	78.94% <ø> (?)
unittests-Instrumentation.Runtime	98.02% <ø> (?)
unittests-Instrumentation.SqlClient	89.58% <ø> (?)
unittests-Instrumentation.StackExchangeRedis	71.21% <ø> (?)
unittests-Instrumentation.Wcf	78.62% <ø> (?)
unittests-PersistentStorage	65.88% <ø> (?)
unittests-Resources.AWS	79.16% <ø> (?)
unittests-Resources.Azure	84.56% <ø> (?)
unittests-Resources.Container	67.34% <ø> (?)
unittests-Resources.Gcp	71.15% <ø> (?)
unittests-Resources.Host	73.91% <ø> (?)
unittests-Resources.OperatingSystem	76.98% <ø> (?)
unittests-Resources.Process	100.00% <ø> (?)
unittests-Resources.ProcessRuntime	78.26% <ø> (?)
unittests-Sampler.AWS	88.23% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...elemetry.Instrumentation.Process/ProcessMetrics.cs	100.00% <ø> (ø)
src/Shared/ActivityHelperExtensions.cs	33.33% <ø> (ø)
src/Shared/ActivityInstrumentationHelper.cs	100.00% <ø> (ø)
src/Shared/DatabaseSemanticConventionHelper.cs	83.33% <ø> (ø)
src/Shared/GrpcTagHelper.cs	54.05% <ø> (ø)
src/Shared/PropertyFetcher.AOT.cs	97.82% <ø> (ø)
src/Shared/RedactionHelper.cs	100.00% <ø> (ø)
src/Shared/RequestDataHelper.cs	81.25% <ø> (ø)

... and 370 files with indirect coverage changes

[CodeBlanch]

LGTM