Clarify process.report glibcVersionRuntime semantics for statically linked glibc builds

What is the issue?

process.report.getReport().header.glibcVersionRuntime can be absent in a Node.js binary that appears to be statically linked against glibc.

That may be intentional, but the current documentation does not appear to define the presence semantics of glibcVersionRuntime, especially for statically linked glibc builds. This makes it easy for ecosystem code to interpret an absent glibcVersionRuntime as "not glibc" or "musl", even when glibcVersionCompiler is present.

Reproduction

Using Node.js v22.21.1:

$ file "$(command -v node)"
/usr/bin/node: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux), statically linked, for GNU/Linux 4.19.0, stripped

$ ldd --version | head -n 1
ldd (GNU libc) 2.39

$ node -p "JSON.stringify({
  node: process.version,
  execPath: process.execPath,
  glibcVersionCompiler: process.report.getReport().header.glibcVersionCompiler,
  glibcVersionRuntime: process.report.getReport().header.glibcVersionRuntime,
  node_target_type: process.config.variables.node_target_type,
  node_shared: process.config.variables.node_shared
}, null, 2)"
{
  "node": "v22.21.1",
  "execPath": "/usr/bin/node",
  "glibcVersionCompiler": "2.39",
  "node_target_type": "static_library",
  "node_shared": false
}

glibcVersionRuntime is omitted from the report object.

Why this is unclear

The diagnostic report documentation shows both fields in the example report:

"glibcVersionRuntime": "2.17",
"glibcVersionCompiler": "2.17"

The report documentation also says the report version is bumped when a key is added or removed, or when a value type changes.

However, I could not find documentation stating whether glibcVersionRuntime is guaranteed to be present on Linux glibc builds, whether it may be omitted, or whether it is expected to be omitted for statically linked glibc builds.

The implementation appears to emit glibcVersionRuntime only when dlsym(RTLD_DEFAULT, "gnu_get_libc_version") succeeds, while glibcVersionCompiler is emitted under __GLIBC__.

Expected documentation / clarification

It would be useful for the documentation to clarify one of the following:

1. glibcVersionRuntime is optional and may be absent when the runtime glibc version cannot be dynamically queried, including statically linked glibc builds; or
2. glibcVersionRuntime should be present for statically linked glibc builds, and its absence is a bug; or
3. consumers should not use absence of glibcVersionRuntime as evidence that the runtime is musl or non-glibc.

I am not asserting that the current runtime behavior is necessarily wrong. The issue is that the field's absence is observable, but the documented contract does not appear to define what that absence means.

Start with the diagnostic report documentation and the process.report.getReport().header fields described in the issue. Clarify whether glibcVersionRuntime may be absent for statically linked glibc and that absence is not evidence of musl; done when the documentation defines its presence semantics alongside the existing example.