Propose a stable metrics API

problem/0000-metrics-v1.md

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+# Linkerd Metrics v1
+
+- Contribution Name: metrics-v1
+- Implementation Owner:
+- Start Date: 2020-04-23
+- Target Date:
+- RFC PR:
+- Linkerd Issue:
+- Reviewers: @grampelberg @adleong
+
+## Problem Statement
+
+[problem-statement]: #problem-statement
+
+Linkerd's `stat` API powers both the command-line and web interfaces, but
+it's tailored closely to their use cases and it has evolved only to express
+the features of these tools. Therefore, anyone trying to access this data
+must access Linkerd's Prometheus API directly. This presents several
+problems:
+
+1. Cluster operators need guidance on how to support Linkerd's Prometheus
+   data. There should be guidance on how to manage cardinality (especially in
+   larger clusters) and how to support multi-tenant environemnts.
+2. Consumers have inadequate documentation and tools to access Linkerd's
+   metrics data. The proxy's metrics export has been treated as an
+   implementation detail and not a public API. Information is exposed in such a
+   way that a consumer essentially has to understand how the proxy is
+   implemented to know how to interpret the data.
+
+The goal of this RFC is to define an external-facing metrics API for Linkerd.
+This API should be stable, versioned, and extensible to support additional
+protocols, metrics, and metadata (via future RFCs).