docs(specs): Add specification for output buffer persistence strategy

docs/specs/tsd-003-output-buffer-strategy.md

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+# Telegraf Output Buffer Strategy
+
+## Objective
+
+Introduce a new agent-level config option to choose a disk buffer strategy for
+output plugin metric queues.
+
+## Overview
+
+Currently when a Telegraf output metric queue fills, either due to incoming
+metrics being too fast or various issues with writing to the output, such as
+connection failures or rate limiting, new metrics are dropped and never
+written to the output. This specification defines a set of options to make
+this output queue more durable by persisting pending metrics to disk rather
+than only an in-memory limited size queue.
+
+## Keywords
+
+output plugins, agent configuration, persist to disk
+
+## Agent Configuration
+
+The configuration is at the agent-level, with options for:
+- **Memory**, the current implementation, with no persistance to disk
+- **Write-through**, all metrics are also written to disk using a
+  Write Ahead Log (WAL) file
+- **Disk-overflow**, when the memory buffer fills, metrics are flushed to a
+  WAL file to avoid dropping overflow metrics
+
+As well as an option to specify a directory to store the WAL files on disk,
+with a default value. These configurations are global, and no change means
+memory only mode, retaining current behavior.
+
+## Metric Ordering and Tracking
+
+Tracking metrics will be accepted either on a successful write to the output
+source like currently, or on write to the WAL file in the case of the
+disk-overflow option. Metrics will be written to their appropriate output in
+the order they are received in the buffer still no matter which buffer
+strategy is chosen.
+
+## Disk Utilization and File Handling
+
+Each output plugin has its own in-memory output buffer, and therefore will
+each have their own WAL file (or potentially files) for buffer persistence.
+Telegraf will not make any attempt to limit the size on disk taken by these
+files, beyond cleaning up WAL files for metrics that have successfully been
+flushed to their output source. It is the user's responsibility to ensure
+these files do not entirely fill the disk, both during Telegraf uptime and
+with lingering files from previous instances of the program.
+
+Telegraf should provide a way to easily flush WAL files from previous
+instances of the program in the event that a crash or system failure
+happens. Telegraf makes no guarantee that in these cases, all metrics will
+be kept. This may be as simple as a plugin which can read these WAL files
+as an input. The file names should be clear to the user what order they are
+in so that if metric order for writing to output is crucial, it can be
+retained. This plugin should not be required for use to allow the buffer
+strategy to work at all, but as a backup option for the user in the event
+that files linger across multiple runs of Telegraf.
+
+## Is/Is-not
+- Is a way to increase the durability of metrics and reduce the potential
+  for metrics to be dropped due to a full in-memory buffer
+- Is not a way to guarantee data safety in the event of a crash or system failure
+- Is not a way to manage file system allocation size, file space will be used
+  until the disk is full
+
+## Prior art
+
+[Initial issue](https://github.com/influxdata/telegraf/issues/802)
+[Loose specification issue](https://github.com/influxdata/telegraf/issues/14805)