Add metadata ADR

adr/ADR-00.md

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+# Metadata for Stream and Consumer
+
+|Metadata|Value|
+|--------|-----|
+|Date    |2023-01-23|
+|Author  |@Jarema|
+|Status  |Proposed|
+|Tags    |jetstream, client, server|
+
+## Context and Problem Statement
+
+Until now, there was no way to easily add additional information about Stream or Consumer.
+The only solution was using `Description` field, which is a not ergonomic workaround.
+
+## Server PR
+https://github.com/nats-io/nats-server/pull/3797
+
+## Design
+
+The solution is to add new `metadata` field to both `Consumer` and `Stream` config.
+The `metadata` field would by a map of `string` keys and `string` values.
+
+### JSON representation
+The map would be represented in json as object with nested key/value pairs, which is a default
+way to marshal maps/hashmaps in most languages.
+
+### Size limit
+To avoid abuse of the metadata, the size of it is limited to 128kB.
+Size is equal to len of all keys and values summed.
+
+### Reserved prefix
+`_nats` is a reserved prefix.
+Will be used for any potential internals of server or clients.
+Server can lock its metadata to be immutable and deny any changes.
+
+
+### Example
+```json
+{
+  "durable_name": "consumer",
+  ... // other consumer/stream fields
+  "metadata": {
+    "owner": "nack",
+    "domain": "product",
+    "_nats_created_version": "1.10.0"
+  }
+}
+
+```
+
+