logJsonDiagnostic

import std.stdio : File, stderr;
import vibe.data.json : Json, parseJsonString;

auto origStderr = stderr;
stderr = File.tmpfile();
scope (exit)
{
    stderr.close();
    stderr = origStderr;
}

logJsonError("error", "mysterious observation", "secret", 42);

stderr.rewind();
auto observed = parseJsonString(stderr.readln);

assert(observed["thread"].type == Json.Type.int_);
assert(observed["timestamp"].type == Json.Type.int_);
assert(observed["error"] == "mysterious observation");
assert(observed["secret"] == 42);