docs(specs): Add specification for partial-write errors

docs/specs/tsd-008-partial-write-error-handling.md

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+# Partial write error handling
+
+## Objective
+
+Provide a way to pass information about partial metric write errors from an
+output to the output model.
+
+## Keywords
+
+output plugins, write, error, output model, metric, buffer
+
+## Overview
+
+The output model wrapping each output plugin buffers metrics to be able to batch
+those metrics for more efficient sending. In each flush cycle, the model
+collects a batch of metrics and hands it over to the output plugin for writing
+through the `Write` method. Currently, if writing succeeds (i.e. no error is
+returned), _all metrics of the batch_ are removed from the buffer and are marked
+as __accepted__ both in terms of statistics as well as in tracking-metric terms.
+If writing fails (i.e. any error is returned), _all metrics of the batch_ are
+__kept__ in the buffer for requeueing them in the next write cycle.
+
+Issues arise when an output plugin cannot write all metrics of a batch bit only
+some to its service endpoint, e.g. due to the metrics being serializable or if
+metrics are selectively rejected by the service on the server side. This might
+happen when reaching submission limits, violating service constraints e.g.
+by out-of-order sends, or due to invalid characters in the serialited metric.
+In those cases, an output currently is only able to accept or reject the
+_complete batch of metrics_ as there is no mechanism to inform the model (and
+in turn the buffer) that only _some_ of the metrics in the batch were failing.
+
+As a consequence, outputs often _accept_ the batch to avoid a requeueing of the
+failing metrics for the next flush interval. This distorts statistics of
+accepted metrics and causes misleading log messages saying all metrics were
+written sucessfully which is not true. Even worse, for outputs ending-up with
+partial writes, e.g. only the first half of the metrics can be written to the
+service, there is no way of telling the model to selectively accept the actually
+written metrics and in turn those outputs must internally buffer the remaining,
+unwritten metrics leading to a duplication of buffering logic and adding to code
+complexity.
+
+This specification aims at defining the handling of partially successful writes
+and introduces the concept of a special _partial write error_ type to reflect
+partial writes and partial serialization overcoming the aforementioned issues
+and limitations.
+
+To do so, the _partial write error_ error type must contain a list of
+successfully written metrics, to be marked __accepted__, both in terms of
+statistics as well as in terms of metric tracking, and must be removed from the
+buffer. Furthermore, the error must contain a list of metrics that cannot be
+sent or serialized and cannot be retried. These metrics must be marked as
+__rejected__, both in terms of statistics as well as in terms of metric
+tracking,  and must be removed from the buffer.
+
+The error may contain a list of metrics not-yet written to be __kept__ for the
+next write cylce. Those metrics must not be marked and must be kept in the
+buffer. If the error does not contain the list of not-yet written metrics, this
+list must be inferred using the accept and reject lists mentioned above.
+
+To allow the model and the buffer to correctly handle tracking metrics ending up
+in the buffer and output the tracking information must be preserved during
+communication between the output plugin, the model and the buffer through the
+specified error. To do so, all metric lists should be communicated as indices
+into the batch to be able to handle tracking metrics correctly.
+
+For backward compatibility and simplicity output plugins can return a `nil`
+error to indicate that __all__ metrics of the batch are __accepted__. Similarly,
+returing an error _not_ being a _partial write error_ indicates that __all__
+metrics of the batch should be __kept__ in the buffer for the next write cycle.
+
+## Related Issues
+
+- [issue #11942](https://github.com/influxdata/telegraf/issues/11942) for
+  contradicting log messages
+- [issue #14802](https://github.com/influxdata/telegraf/issues/14802) for
+  rate-limiting multiple batch sends
+- [issue #15908](https://github.com/influxdata/telegraf/issues/15908) for
+  infinite loop if single metrics cannot be written