Resource detectors: add runtime attributes

[agagniere]

The host, os, and process resource detectors (opt-in via OTEL_EXPERIMENTAL_RESOURCE_DETECTORS) will only populate attributes known at comptime once #159 merges. The following attributes require runtime OS calls and are not yet implemented.

Reference: Python SDK detectors

host detector

• host.name — std.posix.gethostname / GetComputerNameEx on Windows

os detector

• os.version — kernel/release version string (uname -r / RtlGetVersion on Windows)

process detector

• process.pid — std.posix.getpid
• process.parent_pid — std.posix.getppid
• process.executable.name — argv[0] basename
• process.executable.path — argv[0] directory
• process.command — argv[0]
• process.command_line — full command line joined
• process.command_args — argv as a list (repeated string attribute)
• process.owner — username of the process owner

These all require collecting values at provider initialization time rather than comptime, so they need a different code path from the current comptime-only approach in src/sdk/resource.zig.