[log4net] bridge + trace context injection

[lachmatt]

Why

Towards #3822

What

Initial implementation of log4net bridge and injection of trace context.

Bytecode instrumentation for log4net that adds appender exporting logs in OTLP using LogsApi.

Bytecode instrumentation targets and some of the implementation similar to Datadog's log4net DirectLogSubmission/trace context injection implementation. @zacharycmontoya PTAL and approve from Datadog's side.

Logs bridge is disabled by default. In order to enable it, OTEL_DOTNET_AUTO_LOGS_ENABLE_LOG4NET_BRIDGE env variable needs to be set.
Trace context injection is enabled by default.

I added an internal doc with a short description of instrumentation added in this PR, and known limitations.

If *Format overloads (e.g InfoFormat) are used for logging, format string is used as a body of a log record, and args are added as attributes. If additionally OTEL_DOTNET_AUTO_LOGS_INCLUDE_FORMATTED_MESSAGE is set, log4net.rendered_message attribute is added with a content of a formatted message. Sample output:

LogRecord.Timestamp:               2024-12-11T15:03:10.8476757Z
LogRecord.TraceId:                 277283fe7ba44f2e91b0cb0b2d5f5caf
LogRecord.SpanId:                  306922dc6adef3d6
LogRecord.TraceFlags:              Recorded
LogRecord.CategoryName:            TestApplication.Log4NetBridge.Program
LogRecord.Severity:                Info
LogRecord.SeverityText:            INFO
LogRecord.Body:                    {0}, {1} at {2:t}!
LogRecord.Attributes (Key:Value):
    log4net.rendered_message: Hello, world at 16:03!
    0: Hello
    1: world
    2: 12/11/2024 16:03:10

Notes:
Most of the LogsApi from OpenTelemetry .NET SDK is not public in non-rc builds. Due to this, and to achieve acceptable peformance, expressions are used to initialize delegates used for sending logs.

In order to avoid adding multiple appenders serving the same purpose, as a simple workaround for now, this integration is enabled only if ILogger integration is not initialized.

Tests

Included in PR.

Checklist

• CHANGELOG.md is updated.
• Documentation is updated.
• New features are covered by tests.

[lachmatt]

I'm still looking into one problematic scenario, and extracting message template and args that were used for logging, but any feedback at this point would be welcome.

[Kielek]

I would consider moving tests related to the dependencies to the separate package. To avoid brining here transitive dependencies.

[lachmatt]

I added trace context injection, extraction of format string and args. I updated the PR description to better reflect scope of the changes in this PR.

[zacharycmontoya]

I don't see the env variable OTEL_DOTNET_AUTO_LOGS_LOG4NET_INSTRUMENTATION_ENABLED anywhere, only OTEL_DOTNET_AUTO_LOGS_ENABLE_LOG4NET_BRIDGE. Am I missing something?

[Kielek]

We have two levels

1. Injecting context to the log files by standard funtionality https://github.com/open-telemetry/opentelemetry-dotnet-instrumentation/pull/3825/files#diff-77b3519370ff48f6caf7c316f0ece966f2f0871c40e9381d451d2c5b1c76b40fR124
2. Bridging everything to the exporter (typically OTLP) by OTEL_DOTNET_AUTO_LOGS_ENABLE_LOG4NET_BRIDGE

[zacharycmontoya]

Overall LGTM, good job putting together the changes and the testing