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 three
+metrics: `throughput`, `duration`, and `errors`.
+The name of these metrics should be namespaced with the Span category and the Span
+kind.
+
+| Name                               | Type            |
+| ----                               | ----            |
+|`{category}.{span.kind}.throughput` | `Counter`       |
+|`{category}.{span.kind}.duration`   | `ValueRecorder` |
+|`{category}.{span.kind}.errors`     | `Counter`       |
+
+### throughput
+
+The throughput metric represents the total number of operations that occur.
+For each timed operation that occurs, the value of the `throughput` metric should be
+incremented by 1.
+
+### duration
+
+The duration metric represents the total duration of all operations that occur.
+For each timed operation, the value of the `duration` metric should be incremented by
+the duration of the operation in milliseconds.
+
+### errors
+
+The `errors` metric represents the total number of operations that failed.
+For each timed operation that failed, the value of the `errors` metric should be
+incremented by 1.
+
+### Naming examples
+
+A simple HTTP service would create three metrics:
+
+* `http.server.throughput`
+* `http.server.duration`
+* `http.server.errors`
+
+If this service also calls out to other HTTP services, it would create three additional metrics:
+
+* `http.client.throughput`
+* `http.client.duration`
+* `http.client.errors`
+
+If this service also queries one or more databases, it would create three additional metrics:
+
+* `db.client.throughput`
+* `db.client.duration`
+* `db.client.errors`