Required. The value of the header must match
the regular expression specified in regexMatch.
For regular expression grammar, please see:
matching against a port specified in the HTTP
request, use a headerMatch with headerName set
to Host and a regular expression that satisfies
the RFC2616 Host header's port specifier.
This field is a member of oneof_ type.
Required. The name of the HTTP header to
match. For matching against the HTTP request's
authority, use a headerMatch with the header
name ":authority". For matching a request's
method, use the headerName ":method".