Add Status to the Tracing section. (open-telemetry#150)

* Add Status to the Tracing section.

* Mention that canonical codes follow the GRPC ones.

specification/tracing-api.md

@@ -36,6 +36,12 @@ Table of Content
   * [Link creation](#link-creation)
   * [GetContext](#getcontext-1)
   * [GetAttributes](#getattributes)
+* [Status](#status)
+  * [StatusCanonicalCode](#statuscanonicalcode)
+  * [Status creation](#status-creation)
+  * [GetCanonicalCode](#getcanonicalcode)
+  * [GetDescription](#getdescription)
+  * [GetIsOk](#getisok)
 * [SpanData](#spandata)
   * [Constructing SpanData](#constructing-spandata)
   * [Getters](#getters)
@@ -394,6 +400,118 @@ Returns the `SpanContext` of a linked span.
 Returns the immutable collection of attributes associated with this `Link`.
 Order of attributes is not significant.
 
+## Status
+
+`Status` interface represents the status of a finished `Span`. It's composed of
+a canonical code in conjuction with an optional descriptive message.
+
+### StatusCanonicalCode
+
+`StatusCanonicalCode` represents the canonical set of status codes of a finished `Span`, following the [Standard GRPC codes](https://github.com/grpc/grpc/blob/master/doc/statuscodes.md).
+
+#### Ok
+
+The operation completed successfully.
+
+#### Cancelled
+
+The operation was cancelled (typically by the caller).
+
+#### UnknownError
+
+An unknown error.
+
+#### InvalidArgument
+
+Client specified an invalid argument. Note that this differs from `FailedPrecondition`.
+`InvalidArgument` indicates arguments that are problematic regardless of the state of the
+system.
+
+#### DeadlineExceeded
+
+Deadline expired before operation could complete. For operations that change the state
+of the system, this error may be returned even if the operation has completed successfully.
+
+#### NotFound
+
+Some requested entity (e.g., file or directory) was not found.
+
+#### AlreadyExists
+
+Some entity that we attempted to create (e.g., file or directory) already exists.
+
+#### PermissionDenied
+
+The caller does not have permission to execute the specified operation.
+`PermissionDenied` must not be used if the caller cannot be
+identified (use `Unauthenticated1` instead for those errors).
+
+#### ResourceExhausted
+
+Some resource has been exhausted, perhaps a per-user quota, or perhaps
+the entire file system is out of space.
+
+#### FailedPrecondition
+
+Operation was rejected because the system is not in a state required for the operation's
+execution.
+
+#### Aborted
+
+The operation was aborted, typically due to a concurrency issue like sequencer check
+failures, transaction aborts, etc.
+
+#### OutOfRange
+
+Operation was attempted past the valid range. E.g., seeking or reading past end of file.
+Unlike `InvalidArgument`, this error indicates a problem that may be fixed if the system
+state changes.
+
+#### Unimplemented
+
+Operation is not implemented or not supported/enabled in this service.
+
+#### InternalError
+
+Internal errors. Means some invariants expected by underlying system has been broken.
+
+#### Unavailable
+
+The service is currently unavailable. This is a most likely a transient condition
+and may be corrected by retrying with a backoff.
+
+#### DataLoss
+
+Unrecoverable data loss or corruption.
+
+#### Unauthenticated
+
+The request does not have valid authentication credentials for the operation.
+
+### Status creation
+
+API MUST provide a way to create a new `Status`.
+
+Required parameters
+
+- `StatusCanonicalCode` of this `Status`.
+
+Optional parameters
+
+- Description of this `Status`.
+
+### GetCanonicalCode
+
+Returns the `StatusCanonicalCode` of this `Status`.
+
+### GetDescription
+
+Returns the description of this `Status`.
+
+### GetIsOk
+
+Returns false if this `Status` represents an error, else returns true.
+
 ## SpanData
 
 `SpanData` is an immutable and final class. All getters of `SpanData` are thread