Document HBase attributes

[lmolkova]

HBase has native instrumentation in java - https://github.com/apache/hbase/blob/master/hbase-common/src/main/java/org/apache/hadoop/hbase/trace/HBaseSemanticAttributes.java

But hbase semconv does not reflect custom attributes used in the native instrumentation.

[trask]

Moving this to post-stability, I've added comment on Decide on which db.system values to mark stable as part of initial db semconv stability (#965):

I suggest we not include hbase in the initial stability, as it will be good to sync with hbase native instrumentation prior to stabilizing anything related to hbase