hadoop-common-project/hadoop-common/src/site/markdown/Tracing.md: update the documentation to remove references to the obsolete dfs.htrace configuration keys and the deprecated no-argument Tracer#Builder constructor. Update the configuration keys described here to be their modern equivalents.
HdfsClientConfigKeys.java, DFSConfigKeys.java, hdfs-default.xml: remove references to the unused obsolete configuration key dfs.client.htrace.sampler.classes
NameNodeRpcServer.java: we should set the Tracer for the serviceRpcServer (the RPC server which DNs talk to) as well as the for clientRpcServer. Due to a typo, only one of them was getting set.
NamenodeFsck.java: we should construct a Tracer using the configuration prefix namenode.fsck.htrace so that tracing can be enabled specifically for fsck.
TestBlockTokenWithDFS.java, TestDataNodeVolumeFailure.java: set the Tracer object when constructing a BlockReader. It isn't strictly necessary, but it is awkward to have tracer be a null pointer, and it will be helpful if these tests need to be extended in the future. (Note: may need to be done on JspHelper in older branches)