This commit improves the thread safety of OpenCensusTraceContextDataInjector by
following the thread safety requirements in the Javadocs of the overridden
methods from ContextDataInjector.  It also handles the possibility of
ThreadContext.getThreadContextMap() returning null by adding a Nullable
annotation in a Checker Framework stub file and adding null checks to the code.

* Initial import of OpenCensus/Sleuth integration.

* Minor fixes for integration with master.

* Adds a README.

* Removes hardcoded dependencies in build file.

* Adds contrib to spring_sleuth file path.

* Updates package names appropriately based on file renames.

* Fixes build paths.

* Fixes for build checks.

* Mark as experimental.

* Minor fixes for build files (space vs tab).

* Minor fixes for build files (space vs tab).

* Update README to absolve Spring Sleuth team of any responsibility for this spring_sleuth plugin.

* Minor fixes for import control (space vs tab).

* Tag public classes @since 0.16

* Adds javadoc for public methods not already documented.

* Updates OpenCensusSleuthTracer to pass nullness checks.

* Fixes for formatting issues.

* Fixes for typos.

* Suppresses the check return value warning added by gRPC.

* Suppresses the check return value warning added by gRPC.

The stub file adds Nullable annotations to indicate that Context.Key.get can
return null when the key has no default or the key has a nullable value type.

This commit also adds annotations to TextFormat that were required after the
change in checkNotNull.

This commit adds a stub file as described in
https://checkerframework.org/manual/#stub.  The initial file contains
annotations for com.google.common.base.Strings.isNullOrEmpty.