Callback function invoked when the reception of header block in
HEADERS or PUSH_PROMISE is started. Each header header field
will be emitted by Connector.onHeaderField`.
The `frame.hd.flags` may not have
FrameFlags.END_HEADERS flag set, which indicates that one
or more CONTINUATION frames are involved. But the application does
not need to care about that because the header fields are
emitted transparently regardless of CONTINUATION frames.
If this callback returns false, stream fails with ErrorCode.CALLBACK_FAILURE
Callback function invoked when the reception of header block in HEADERS or PUSH_PROMISE is started. Each header header field will be emitted by Connector.onHeaderField`.
The `frame.hd.flags` may not have FrameFlags.END_HEADERS flag set, which indicates that one or more CONTINUATION frames are involved. But the application does not need to care about that because the header fields are emitted transparently regardless of CONTINUATION frames.
If this callback returns false, stream fails with ErrorCode.CALLBACK_FAILURE