[broker][monitoring] Support gzip compression when expose metrics data

[tjiuming]

Motivation

support gzip compression when expose metrics data

Modifications

1. add a configuration: enableMetricsDataCompression

Documentation

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[merlimat]

I think GZIPOutputStream will create a lot of garbage on the JVM heap :|

Could we just use org.apache.pulsar.common.compression.CompressionCodecZLib on the buffer?

[tjiuming]

@merlimat But CompressionCodecZLib not in GZIP format, and Prometheus read metrics data in GZIP format:

	if s.gzipr == nil {
		s.buf = bufio.NewReader(resp.Body)
		s.gzipr, err = gzip.NewReader(s.buf)
		if err != nil {
			return "", err
		}
	} else {
		s.buf.Reset(resp.Body)
		if err = s.gzipr.Reset(s.buf); err != nil {
			return "", err
		}
	}

	n, err := io.Copy(w, io.LimitReader(s.gzipr, s.bodySizeLimit))
	s.gzipr.Close()
	if err != nil {
		return "", err
	}