* Ignore null or blank string-valued span attributes.
* also ignore null & blank strings on active span attributes.
* change to ignore at the AttributeValue level, rather than above
* Rename tags to dctx (and related members).
* Implement the renamed dctx layer for the Sdk.
* Fix inline doc.
* Rename dctx to distributedcontext.
* Rename distributedcontext Attribute* to Entry*.
* Rename dctx to distributedcontext in the Sdk.
* Update the recently added SDK classes.
Giovanni Liva
Armin Ruech
John Watson
Bogdan Drutu
Carlos Alberto Cortez