Add metrics semantic conventions for timed operations

specification/metrics/semantic_conventions/metric-general.md

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+# General
+
+The conventions described in this section are generic; they are not specific to a
+particular operation.
+
+## Summarizing timed operations
+
+Timed operations that may be more fully described by Spans can be summarized by a single [`ValueRecorder`](../api.md#valuerecorder) recording duration.  The instrument name should be prefixed with the Span category and kind.
+The resulting name should look like `{category}.{span.kind}.duration`. See [examples](#examples) below.
+
+For each operation being recorded, the duration of the operation **in milliseconds** should be recorded onto the `duration` instrument.
+
+#### errors
+
+The `duration` metric should include an `error` label indicating whether the operation failed.  The value of this label should be `true` if the operation ended in a failed state.
+
+### Examples
+
+A simple HTTP service would create the metric:
+
+* `http.server.duration`
+
+If this service also calls out to other HTTP services, it would create the additional metric:
+
+* `http.client.duration`
+
+If this service also queries one or more databases, it would create the additional metric:
+
+* `db.client.duration`
+
+All of these metrics would include labels
+  * `error=true`
+  * `error=false`