SDK Trace: Adding span collection limits

Fixes open-telemetry#182

To help safeguard against a common error, adding default limits
to the number of events per span in the SDK.

spec-compliance-matrix.md

@@ -14,6 +14,8 @@ status of the feature is not known.
 |[TracerProvider](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/api.md#tracerprovider-operations)|
 |Create TracerProvider                         | + | +  | + | +    | +  | +    | + | +  | + | +  |
 |Get a Tracer                                  | + | +  | + | +    | +  | +    | + | +  | + | +  |
+|Set a collection limit                        |   |    |   |      |    |      |   |    |   |    |
+|Get a collection limit                        |   |    |   |      |    |      |   |    |   |    |
 |Safe for concurrent calls                     | + | +  | + |      | +  | +    | + | +  | + | +  |
 |[Tracer](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/api.md#tracer-operations)|
 |Create a new Span                             | + | +  | + | +    | +  | +    | + | +  | + | +  |
@@ -37,6 +39,8 @@ status of the feature is not known.
 |IsRecording                                   | + | +  | + | +    | +  | +    | + |    | + | +  |
 |Set status                                    | + | +  | + | +    | +  | +    | + | +  | + | +  |
 |Safe for concurrent calls                     | + | +  | + |      | +  | +    | + | +  | + | +  |
+|Set a collection limit                        |   |    |   |      |    |      |   |    |   |    |
+|Discard events greater than collection limit  |   |    |   |      |    |      |   |    |   |    |
 |[Span attributes](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/api.md#set-attributes)|
 |SetAttribute                                  | + | +  | + | +    | +  | +    | + | +  | + | +  |
 |Set order preserved                           | + | -  | + | +    | +  | +    | + | +  | + | +  |

specification/trace/sdk.md

@@ -236,7 +236,27 @@ Thus, the SDK specification defines sets of possible requirements for
   that the [span creation API](api.md#span-creation) returned (or will return) to the user
   (for example, the `Span` could be one of the parameters passed to such a function,
   or a getter could be provided).
-
+
+## Span Event Limits
+
+Erroneous code can add unintended events to a span. If these containers are
+unbounded, they can quickly exhaust available memory, resulting in crashes that
+are difficult to recover from safely.
+
+To protect against such errors, SDK Spans MUST discard attributes, links, and
+events that would increase the size of each collection beyond a limit configured
+in the `TracerProvider`, or for the span specifically.
+
+There MUST be a log message emitted by the SDK for each object discarded.
+
+The following interfaces MUST exist:
+
+- `TracerProvider`: a method to set a maximum collection size
+- `TracerProvider`: a method to retrieve the maximum collection size
+- `Span`: a method to set a maximum collection size for the span
+
+The default for this limit SHOULD be 1000. A value of 0 indicates no limit.
+
 ## Span processor
 
 Span processor is an interface which allows hooks for span start and end method