A proposal for SDK configurations for metric aggregation (Basic Views) (

open-telemetry#126)

* Add a proposal for SDK configurations for metric aggregation.

* rename file to match the PR #

* fix markdown lint issues

* clarify that this applies to the default SDK, and fill out the open questions

* update the java example to fix the naming changes

* Add another open question to the list.

* Update text/0126-Configurable-Metric-Aggregations.md

Co-authored-by: Chris Kleinknecht <libc@google.com>

* Update to use the 'view' terminology

* another configuration/view replacement

* Add a few more open questions, and a note that they will be resolved in the spec.

* Update text/0126-Configurable-Metric-Aggregations.md

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

Co-authored-by: Chris Kleinknecht <libc@google.com>
Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>
Co-authored-by: Bogdan Drutu <bogdandrutu@gmail.com>

text/0126-Configurable-Metric-Aggregations.md

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+# A Proposal For SDK Support for Configurable Batching and Aggregations (Basic Views)
+
+Add support to the default SDK for the ability to configure Metric Aggregations.
+
+## Motivation
+
+OpenTelemetry's architecture separates the concerns of instrumentation and operation. The Metric Instruments
+provided by the Metric API are all defined to have a default aggregation. And, by default, aggregations are
+performed with all Labels being used to define a unit of aggregation. Although this is a good default
+configuration for the SDK to provide, more configurability is needed.
+
+There are 3 main use-cases that this proposal is intended to address:
+
+1) The application developer/operator wishes to use an aggregation other than the default provided by the SDK
+for a given instrument or set of instruments.
+2) An exporter author wishes to inform the SDK what "Temporality" (delta vs. cumulative) the resulting metric
+data points represent. "Delta" means only metric recordings since the last reporting interval are considered
+in the aggregation, and "Cumulative" means that all metric recordings over the lifetime of the Instrument are
+considered in the aggregation.
+3) The application developer/operator wishes to constrain the cardinality of labels for metrics being reported
+to the metric vendor/backend of choice.
+
+## Explanation
+
+I propose a new feature for the default SDK, available on the interface of the SDK's MeterProvider implementation, to configure
+the batching strategies and aggregations that will be used by the SDK when metric recordings are made. This is the beginnings
+of a "Views" API, but does not intend to implement the full View functionality from OpenCensus.
+
+The basic API has two parts.
+
+* InstrumentSelector - Enables specifying the selection of one or more instruments for the configuration to apply to.
+  - Selection options include: the instrument type (Counter, ValueRecorder, etc), and a regex for instrument name.
+  - If more than one option is provided, they are considered additive.
+  - Example: select all ValueRecorders whose name ends with ".duration".
+* View - configures how the batching and aggregation should be done.
+  - 3 things can be specified: The aggregation (Sum, MinMaxSumCount, Histogram, etc), the "temporality" of the batching,
+    and a set of pre-defined labels to consider as the subset to be used for aggregations.
+    - Note: "temporality" can be one of "DELTA" and "CUMULATIVE" and specifies whether the values of the aggregation
+      are reset after a collection is done or not, respectively.
+  - If not all are specified, then the others should be considered to be requesting the default.
+  - Examples:
+    - Use a MinMaxSumCount aggregation, and provide delta-style batching.
+    - Use a Histogram aggregation, and only use two labels "route" and "error" for aggregations.
+    - Use a quantile aggregation, and drop all labels when aggregating.
+
+In this proposal, there is only one View associated with each selector.
+
+As a concrete example, in Java, this might look something like this:
+
+```java
+ // get a handle to the MeterSdkProvider (note, this is concrete name of the default SDK class in java, not a general SDK)
+ MeterSdkProvider meterProvider = OpenTelemetrySdk.getMeterProvider();
+
+ // create a selector to select which instruments to customize:
+ InstrumentSelector instrumentSelector = InstrumentSelector.newBuilder()
+  .instrumentType(InstrumentType.COUNTER)
+  .build();
+
+ // create a configuration of how you want the metrics aggregated:
+ View view =
+      View.create(Aggregations.minMaxSumCount(), Temporality.DELTA);
+
+ //register the configuration with the MeterSdkProvider
+ meterProvider.registerView(instrumentSelector, view);
+```
+
+## Internal details
+
+This OTEP does not specify how this should be implemented in a particular language, only the functionality that is desired.
+
+A prototype with a partial implementation of this proposal in Java is available in PR form [here](https://github.com/open-telemetry/opentelemetry-java/pull/1412)
+
+## Trade-offs and mitigations
+
+This does not intend to deliver a full "Views" API, although it is the basis for one. The goal here is
+simply to allow configuration of the batching and aggregation by operators and exporter authors.
+
+This does not intend to specify the exact interface for providing these configurations, nor does it
+consider a non-programmatic configuration option.
+
+## Prior art and alternatives
+
+* Prior Art is probably mostly in the [OpenCensus Views](https://opencensus.io/stats/view/) system.
+* Another [OTEP](https://github.com/open-telemetry/oteps/pull/89) attempted to address building a Views API.
+
+## Open questions (to be resolved in an official specification)
+
+1. Should custom aggregations be allowable for all instruments? How should an SDK respond to a request for a non-supported aggregation?
+2. Should the requesting of DELTA vs. CUMULATIVE be only available via an exporter-only API, rather than generally available to all operators?
+3. Is regex-based name matching too broad and dangerous? Would the alternative (having to know the exact name of all instruments to configure) be too onerous?
+4. Is there anything in this proposal that would make implementing a full Views API (i.e. having multiple, named aggregations per instrument) difficult?
+5. How should an exporter interact with the SDK for which it is configured, in order to change aggregation settings?
+6. Should the first implementation include label reduction, or should that be done in a follow-up OTEP/spec?
+7. Does this support disabling an aggregation altogether, and if so, what is the interface for that?
+8. What is the precedence of selectors, if more than one selector can apply to a given Instrument?
+
+## Future possibilities
+
+What are some future changes that this proposal would enable?
+
+- A full-blown views API, which would allow multiple "views" per instrument. It's unclear how an exporter would specify which one it wanted, or if it would all the generated metrics.
+- Additional non-programmatic configuration options.