Simplify log configuration variables and include docs for no-code troubleshooting

[stevejgordon]

It's currently possible to enable logging when using the no-code startup hooks by configuring the following two env vars:

• ELASTIC_APM_PROFILER_LOG, which takes the log level to use
• ELASTIC_APM_PROFILER_LOG_DIR, which defines where profiler logs (and potentially agent logs are written)

Due to implementation changes we have previously made, when no logger is set in AgentComponents, the profiler checks fallback logs to a file in the provided directory. This behaviour is helpful, especially for no-code scenarios, but the environment variable names are confusing. We also have ELASTIC_APM_LOG_LEVEL, which doesn't apply in the no-code case.

We should rename ELASTIC_APM_PROFILER_LOG_DIR to ELASTIC_APM_LOG_DIRECTORY and, when set, use this for all file-based logging, including startup hooks logging.

We should remove ELASTIC_APM_PROFILER_LOG and always use the more descriptive ELASTIC_APM_LOG_LEVEL variable for configuring the overall log level of all logs (agent and profiler). The profiler also listens to ELASTIC_APM_LOG_LEVEL and currently prefers the more granular of the two.

When ELASTIC_APM_LOG_DIRECTORY is set, even with another logging provider (e.g., ILogger) enabled, we should consider whether we also log to a file in this directory.

These changes will simplify troubleshooting and make gathering all relevant logs for support scenarios easier.

We should ensure all docs concerning these environment variables are updated, including ELASTIC_APM_STARTUP_HOOKS_LOGGING,