feat(logs): add log4net integration

[Flash0ver]

closes #4731

Changes

Add log4net integration to Sentry Structured Logging.

Docs

• docs(dotnet/logs): add log4net structured logs integration sentry-docs#18739

See also

• fix: populate Default attributes on structured logs from current scope #5216

Implementation Notes

Sentry's log4net support is a log4net appender (SentryAppender : AppenderSkeleton). Every log call flows through Append(LoggingEvent).

Previously each event became either a SentryEvent (error) or a breadcrumb. Now, when options.EnableLogs is on, Append also calls CaptureStructuredLog to emit a Sentry structured log.

log4net-specific stuff

1. Level mapping. log4net doesn't have a clean enum — it has ~18 named Level objects (All, Finest, Verbose, Finer, Trace, Fine, Debug, Info, Notice, Warn, Error, Severe, Critical, Alert, Fatal, Emergency, …) plus user-defined custom levels with arbitrary integer values. ToSentryLogLevel collapses all that into Sentry's 6 levels using >= threshold comparisons on the numeric value, so custom levels land in the nearest bucket and Level.Off drops the log (returns null).

2. Message: rendered only. log4net pre-renders the final string into LoggingEvent.RenderedMessage, which becomes the log Message.

3. No template, no parameters. log4net can't give us the original template of parameters:

const string? template = null;   // cannot get format-string from log4net.Util.SystemStringFormat
var parameters = ImmutableArray<...>.Empty; // cannot get arguments either

log4net's message object is typically a SystemStringFormat that keeps the composite format string ("{0} {1}") and the args array as private fields with no public accessor. This is a limitation of log4net rather than Sentry's integration.

4. Properties → property.* attributes, with log4net-specific filtering. log4net merges event-level properties, ThreadContext.Properties, GlobalContext.Properties, and its own metadata into one bag via GetProperties(). The loop (Structured.cs:20-29) maps each DictionaryEntry to a property.<key> attribute, skipping:

• empty keys,
• keys starting with log4net: (log4net's internal entries — HostName, Identity, etc.),
• GUID-shaped keys (log4net's transient/internal correlation entries).

This property.<key> convention matches Serilog, not MEL. (MEL doesn't have an ambient property bag — it emits parameters plus category.name/event.id/event.name.)

[codecov]

Codecov Report

❌ Patch coverage is 83.82353% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.61%. Comparing base (b006e95) to head (4236d5f).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Sentry.Log4Net/SentryAppender.cs	80.76%	3 Missing and 2 partials ⚠️
src/Sentry.Log4Net/LevelMapping.cs	75.00%	1 Missing and 2 partials ⚠️
src/Sentry.Log4Net/SentryAppender.Structured.cs	86.36%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5172      +/-   ##
==========================================
+ Coverage   74.23%   74.61%   +0.37%     
==========================================
  Files         509      512       +3     
  Lines       18435    18540     +105     
  Branches     3610     3617       +7     
==========================================
+ Hits        13685    13833     +148     
+ Misses       3875     3834      -41     
+ Partials      875      873       -2     

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[jamescrosswell]

@Flash0ver I put this back in draft state until CI passes. Looks like there is some warden feedback worth checking as well.