Add server-timing as a trailer

[mitar]

It seems support for server-timing header as a trailer over HTTP2 is pretty lacking across browsers. Chrome does not detect it and does not show it in web inspector.

[mitar]

It seems this is a regression: https://bugs.chromium.org/p/chromium/issues/detail?id=1280260

[Schweinepriester]

Related: #2482