Clarify HTTP client duration

CHANGELOG.md

@@ -68,3 +68,5 @@ release.
   ([#13](https://github.com/open-telemetry/semantic-conventions/pull/13))
 - Clarify `process.runtime.jvm.threads.count` refers to platform threads.
   ([#54](https://github.com/open-telemetry/semantic-conventions/pull/54))
+- Clarify when HTTP client spans should end.
+  ([#70](https://github.com/open-telemetry/semantic-conventions/pull/70))

specification/trace/semantic_conventions/http.md

@@ -153,6 +153,13 @@ This span type represents an outbound HTTP request. There are two ways this can
 
 For an HTTP client span, `SpanKind` MUST be `Client`.
 
+HTTP client spans SHOULD end sometime after all the response headers are received. This MAY or MAY NOT include time spent reading the response body.
+
+HTTP client library instrumentations SHOULD document their behavior around ending HTTP client spans.
+
+> **Note**
+> It is nice for HTTP client spans to end once the response body has been fully read. However, if there is any possibility for application code to not fully read the HTTP response (and for the HTTP client library to then have to clean up the HTTP response asynchronously), the HTTP client span SHOULD be ended earlier (e.g. after the HTTP response headers are read). This avoids having to end the span asynchronously later on at a time which is no longer directly associated with the application code which made the HTTP request.
+
 <!-- semconv trace.http.client(full) -->
 | Attribute  | Type | Description  | Examples  | Requirement Level |
 |---|---|---|---|---|