$ java src/OptIn.java https://localhost:4433/hello client     (OpenJDK Runtime Environment Temurin-26.0.2.1+1 (build 26.0.2.1+1))
client-level HTTP_3, request 1: HTTP_2
client-level HTTP_3, request 2: HTTP_3
client-level HTTP_3, request 3: HTTP_3
client-level HTTP_3, request 4: HTTP_3

$ java src/OptIn.java https://localhost:4433/hello request    (OpenJDK Runtime Environment Temurin-26.0.2.1+1 (build 26.0.2.1+1))
request-level HTTP_3, request 1: HTTP_3
request-level HTTP_3, request 2: HTTP_3
request-level HTTP_3, request 3: HTTP_3
request-level HTTP_3, request 4: HTTP_3

$ java src/OptIn.java https://localhost:4433/hello client     (OpenJDK Runtime Environment Temurin-25.0.4.1+1 (build 25.0.4.1+1-LTS))
src/OptIn.java:12: error: cannot find symbol
        if (where.equals("client")) builder.version(HttpClient.Version.HTTP_3);
                                                                      ^
  symbol:   variable HTTP_3
  location: class Version
src/OptIn.java:15: error: cannot find symbol

$ first request of a fresh client-level HTTP_3 client, 8 fresh JVMs (OpenJDK Runtime Environment Temurin-26.0.2.1+1 (build 26.0.2.1+1))
client-level HTTP_3, request 1: HTTP_2
client-level HTTP_3, request 1: HTTP_2
client-level HTTP_3, request 1: HTTP_2
client-level HTTP_3, request 1: HTTP_2
client-level HTTP_3, request 1: HTTP_2
client-level HTTP_3, request 1: HTTP_2
client-level HTTP_3, request 1: HTTP_2
client-level HTTP_3, request 1: HTTP_2