Add metric semantic conventions for timed operations

text/metrics/0129-semantic-conventions-for-timed-operations.md

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+# Semantic Conventions for Timed Operations
+
+## Motivation
+
+Timed operations are generally fully described by Spans. Because these Spans may be
+sampled, aggregating information about them downstream is subject to inaccuracies.
+Computing count, duration, and error rate from _all_ operations, regardless of whether
+the result in a sampled Span, results in more accurate aggregates.
+
+## Explanation
+
+Summarization of timed operations may be done with a single `ValueRecorder`, onto which
+the duration of each instance of the timed operation is recorded.
+
+### Naming
+
+The semantic conventions should specify the naming of this metric instrument, following
+the [Metric Instrument Naming Guidelines](./0108-naming-guidelines.md#guidelines).
+
+The goal of the semantic conventions regarding the naming of this metric instrument will
+be that similar operations are rolled up together.
+
+### Labels
+
+The attributes described in the existing tracing semantic conventions provide guidance
+for the labels to be added to this metric instrument. Some of these attributes will
+result in very high cardinality; the semantic conventions should describe which attributes
+should be included and which should be omitted or reformatted to reduce cardinality.
+
+In addition, the tracing attributes allow more value types than metric instrument labels,
+so semantic conventions must describe how to represent those values as labels.