[VAULT-33970] Ensure logger writes to log_file by preventing overwrite of InterceptLogger's output

[elliesterner]

Description

This fixes a bug with how vault handles logging to the log_file in configuration.

Initially, ServerCommand.run sets up a gated writer to standard error, which is passed through configureLogging to loghelper.Setup. This setup combines the gated writer with a file (log_file) using a multiwriter, ensuring logs go to both destinations. Later in server.Run, vault flushes the gated writer by calling the function ResetOutputWithFlush. Unfortunately, this replaces the InterceptLogger’s output with a single writer that logs only to standard error. Some logs still get written to both destinations. This is likely because vault defines some subloggers (like core) before the flush occurs. Naming a sublogger creates a copy of the underlying writer, which remains connected to the original multiwriter.

As a result, loggers created before the flush continue to log to both standard error and the file, while the base logger’s writer gets reset to standard error. This PR removes the logic that replaces the InterceptLogger's output, and instead just flushes the gated writer.

I did some digging into why we introduced the ResetOutputWithFlush function in the first place and found this PR. Originally, it seems we replaced the gated writer with regular standard err to avoid a dealing with mutexes, but right now we're just replacing the multiwriter (which includes the gated writer) with another gated writer, so I don't think we're reaping the benefit anymore. Simply flushing the gated writer also matches what we do in agent and proxy.

TODO only if you're a HashiCorp employee

• Backport Labels: If this fix needs to be backported, use the appropriate backport/ label that matches the desired release branch. Note that in the CE repo, the latest release branch will look like backport/x.x.x, but older release branches will be backport/ent/x.x.x+ent.
  • LTS: If this fixes a critical security vulnerability or severity 1 bug, it will also need to be backported to the current LTS versions of Vault. To ensure this, use all available enterprise labels.
• ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature
  of a public function, even if that change is in a CE file, double check that
  applying the patch for this PR to the ENT repo and running tests doesn't
  break any tests. Sometimes ENT only tests rely on public functions in CE
  files.
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