HTTP duration metrics should match HTTP span duration (#69)

CHANGELOG.md

@@ -70,3 +70,5 @@ release.
   ([#54](https://github.com/open-telemetry/semantic-conventions/pull/54))
 - Add `gcp.gce.instance.name` and `gcp.gce.instance.hostname` resource
   attributes for GCP Compute Engine VMs. ([#15](https://github.com/open-telemetry/semantic-conventions/pull/15))
+- Add note that HTTP duration metrics should match HTTP span duration.
+  ([#69](https://github.com/open-telemetry/semantic-conventions/pull/69))

specification/metrics/semantic_conventions/http-metrics.md

@@ -56,6 +56,8 @@ operations. By adding HTTP attributes to metric events it allows for finely tune
 
 This metric is required.
 
+When this metric is reported alongside an HTTP server span, the metric value SHOULD be the same as the HTTP server span duration.
+
 This metric SHOULD be specified with
 [`ExplicitBucketBoundaries`](https://github.com/open-telemetry/opentelemetry-specification/tree/v1.21.0/specification/metrics/api.md#instrument-advice)
 of `[ 0, 0.005, 0.01, 0.025, 0.05, 0.075, 0.1, 0.25, 0.5, 0.75, 1, 2.5, 5, 7.5, 10 ]`.
@@ -241,6 +243,8 @@ SHOULD NOT be set if only IP address is available and capturing name would requi
 
 This metric is required.
 
+When this metric is reported alongside an HTTP client span, the metric value SHOULD be the same as the HTTP client span duration.
+
 This metric SHOULD be specified with
 [`ExplicitBucketBoundaries`](https://github.com/open-telemetry/opentelemetry-specification/tree/v1.21.0/specification/metrics/api.md#instrument-advice)
 of `[ 0, 0.005, 0.01, 0.025, 0.05, 0.075, 0.1, 0.25, 0.5, 0.75, 1, 2.5, 5, 7.5, 10 ]`.