I'm using Spring Boot 2.2.11 with Tomcat 9.0.39. I found when compression is enabled, the bytesSent metric exposed by org.apache.coyote.RequestGroupInfo is always 0. Those metrics are exposed in JMX as well.

I believe the issue is caused by org.apache.coyote.http11.filters.GzipOutputFilter.doWrite(ByteBuffer) implementation. It doesn't change the chunk parameter position in this code branch:

    if (chunk.hasArray()) {
        compressionStream.write(chunk.array(), chunk.arrayOffset() +  chunk.position(), len);
    } 

So org.apache.coyote.Response.doWrite(ByteBuffer) method will not be able to update contentWritten correctly:

    contentWritten += len - chunk.remaining();