Enable global file logging

[Mpdreamz]

This ensures we can specify file logging globally and uniformly. Greatly simplifying the debug-ability of the agent.

The preferred way is to set any or all of the following:

• ELASTIC_OTEL_FILE_LOG_LEVEL (to anything but none)
• ELASTIC_OTEL_FILE_LOG_DIRECTORY
• ELASTIC_OTEL_LOG_TARGETS (to anything but none)
  • if only set to stdout no file will be created but global logging will kicking )

This ensure we have one way to debug both our proprietary agent as well as the Elastic OpenTelemetry Distribution.

For backwards compatible reasons the profiler variables are also supported:

• ELASTIC_APM_PROFILER_LOG
• ELASTIC_APM_PROFILER_LOG_DIR
• ELASTIC_APM_PROFILER_LOG_TARGETS

Globally setting

• ELASTIC_APM_LOG_LEVEL and ELASTIC_APM_LOG_DIRECTORY is also supported but not preferred.

Setting these in any of our supported deploy scenarios:

• Manual instrumentation (nuget)
  • ASP.NET (classic)
  • ASP.NET core
    • NOTE: we now log to both the configured ILogger and our global logging.
• Auto Instrumentation
  • Profiler
  • Startup hooks To keep support existing deploys this now always globally logs to file if only ELASTIC_APM_STARTUP_HOOKS_LOGGING is set as well.

This further updates our docs for the profiler and troubleshooting to prefer ELASTIC_OTEL_* variables.

The profiler is updated to read the same environment variables as managed code.