Make defaultUseHttp2Preface flag more explicit

[ikhoon]

defaultUseHttp2Preface has only true and false options and does not tell which protocols HTTP/2 connection preface should be used for. Currently, the option is applied to both http and h2c. It is not supported to apply either http or h2c.

If we improve defaultUseHttp2Preface and allow users to specify protocols, it would be helpful to troubleshoot various network issues.

Proposal:

• Syntax: defaultUseHttp2Preface=string|array
• Example
  • defaultUseHttp2Preface=all
    • Always use HTTP/2 even for h1c. This option may be used to diagnose network issues.
    • Generally, it is not recommended.
  • defaultUseHttp2Preface=http,h2c
    • Use connection preface for HTTP and H2C
    • This value can be the sensible default.
  • defaultUseHttp2Preface=h2c
    • Connection preface is only used for H2C.
    • This would be a good option if you have to communicate with legacy servers that do not understand HTTP/2 upgrade requests.
  • defaultUseHttp2Preface=none
    • Always use an upgrade request
• Backward compatibility
  • defaultUseHttp2Preface=true
    • true means http,h2c
  • defaultUseHttp2Preface=false
    • false means none