http: add HTTP response text to the diagnostic messages

[bazsi]

This patch adds the response text returned by the HTTP server to various diagnostic messages, so issues with the server can be diagnosed more easily.

@version: current

options { keep-hostname(yes); };

log {
	source { tcp(port(2000)); };

	destination {
		http(
			#url("https://httpbin.org/status/502")
			url("https://httpbin.org/html")
			content-compression(gzip)
			workers(1)
			tls(
				ssl_version(tlsv1_2)
				peer-verify(no)
			)
			time-reopen(1)
			body("$DATE $HOST $MSGHDR$MSG")
			batch-bytes(10MiB)
			batch-timeout(1000)
			retries(3)
		); 
	};
};

[MrAnno]

.

[MrAnno]

A few of these messages are on the "notice" level, not on debug.

Nevertheless, they seem useful and hopefully they are triggered rarely under normal circumstances.