Remove the Metric Gauge instrument, recommend use of other instruments

text/0080-remove-metric-gauge.md

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+# Remove the Metric API Gauge instrument
+
+The [Observer instrument](./0072-metric-observer.md) is semantically
+identical to the metric Gauge instrument, only it is reported via a
+callback instead of synchronous API calls.  Implementation has shown
+that Gauge instruments are difficult to reason about because the
+semantics of a "last value" Aggregator have to address questions about
+statefulness--the SDK's ability to recall old values.  Observer
+instruments avoid some of these concerns because they are reported
+once per collection period, making it easier to reason about "all
+values" in an aggregator.
+
+## Motivation
+
+Observer instruments improve on our ability to define certain
+aggregations, compared with the existing Gauge instrument.  Because of
+callbacks, Observer instruments support reporting a complete snapshot
+of the "last" values of an instrument, for all active label sets, for
+a single collection period.  This is accomplished requiring new state
+to be managed by the SDK, simply because the Observer instrument is
+expected to support a current value for all active label sets.
+
+## Explanation
+
+Knowing that each collection period contains a complete set of
+instrument values, it is possible to define a sum aggregation and to
+assign ratios to values.  This aggregation is not possible for Gauge
+instruments without imposing new state requirements on the SDK,
+essentially requiring it to remember values from prior collection
+periods.
+
+## Internal details
+
+The Gauge instrument will be removed from the specification at the
+same time the Observer instrument is added.  This will make the
+transition easier because in many cases, Observer instruments simply
+replace Gauge instruments in the text.
+
+## Trade-offs and mitigations
+
+Not much is lost to the user from removing Gauge instruments.  There
+may be situations where it is undesirable to be interrupted by the
+Metric SDK in order to execute an Observer callback--situations where
+Observer semantics are correct but a synchronous API is more
+acceptable.  These cases can be addressed by Observer instruments
+paired with helpers to maintain last-value state and define the
+"complete" set of values.  This requires more of the user, but users
+are likely to be able to optimize this more than the SDK could have,
+and this forces the user to recognize that uses of Gauge-like
+instruments generally should know the complete set of values when
+reporting.
+
+## Prior art and alternatives
+
+Many existing Metric libraries support both synchronous and
+asynchronous Gauge-like instruments.
+
+See the initial discussion in [Spec issue
+412](https://github.com/open-telemetry/opentelemetry-specification/issues/412).