Storage Extension 1/N (open-telemetry#2883)

* WIP - Add storage extension framework

* Cleaned up initial implmenetation. Still much to do

* Remove extraneous dependency

* go tidy

* rm gopls from go.mod

* Fix typo in TODO

* Finish thought in readme

* Document storage.Extension, fix doc issues

* Run make gendependabot and make gotidy

* Use core nop pattern, add context to methods, clean up comments

.github/dependabot.yml

@@ -129,6 +129,10 @@ updates:
     directory: "/extension/observer/k8sobserver"
     schedule:
       interval: "weekly"
+  - package-ecosystem: "gomod"
+    directory: "/extension/storage"
+    schedule:
+      interval: "weekly"
   - package-ecosystem: "gomod"
     directory: "/internal/aws"
     schedule:

extension/storage/Makefile

@@ -0,0 +1 @@
+include ../../Makefile.Common

extension/storage/README.md

@@ -0,0 +1,19 @@
+# Storage
+
+A storage extension persists state beyond the collector process. Other components can request a storage client from the storage extension and use it to manage state. 
+
+The `storage.Extension` interface extends `component.Extension` by adding the following method:
+```
+GetClient(component.Kind, configmodels.NamedEntity) (Client, error)
+```
+
+The `storage.Client` interface contains the following methods:
+```
+Get(string) ([]byte, error)
+Set(string, []byte) error
+Delete(string) error
+```
+Note: All methods should return error only if a problem occurred. (For example, if a file is no longer accessible, or if a remote service is unavailable.)
+
+# TODO Sample code
+- Document component expectations

extension/storage/go.mod

@@ -0,0 +1,10 @@
+module github.com/open-telemetry/opentelemetry-collector-contrib/extension/storage
+
+go 1.14
+
+require (
+	github.com/stretchr/testify v1.7.0
+	go.etcd.io/bbolt v1.3.3
+	go.opentelemetry.io/collector v0.23.0
+	go.uber.org/zap v1.16.0
+)

extension/storage/nop_client.go

@@ -0,0 +1,41 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package storage
+
+import "context"
+
+type nopClient struct{}
+
+var nopClientInstance Client = &nopClient{}
+
+// NewNopClient returns a nop client
+func NewNopClient() Client {
+	return nopClientInstance
+}
+
+// Get does nothing, and returns nil, nil
+func (c nopClient) Get(context.Context, string) ([]byte, error) {
+	return nil, nil // no result, but no problem
+}
+
+// Set does nothing and returns nil
+func (c nopClient) Set(context.Context, string, []byte) error {
+	return nil // no problem
+}
+
+// Delete does nothing and returns nil
+func (c nopClient) Delete(context.Context, string) error {
+	return nil // no problem
+}

extension/storage/storage.go

@@ -0,0 +1,49 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package storage
+
+import (
+	"context"
+
+	"go.opentelemetry.io/collector/component"
+	"go.opentelemetry.io/collector/config/configmodels"
+)
+
+// Extension is the interface that storage extensions must implement
+type Extension interface {
+	component.Extension
+
+	// GetClient will create a client for use by the specified component.
+	// The component can use the client to manage state
+	// TODO change parameters to new config.ComponentID
+	// https://github.com/open-telemetry/opentelemetry-collector/pull/2869
+	GetClient(context.Context, component.Kind, configmodels.NamedEntity) (Client, error)
+}
+
+// Client is the interface that storage clients must implement
+// All methods should return error only if a problem occurred
+type Client interface {
+
+	// Get will retrieve data from storage that corresponds to the
+	// specified key. It should return nil, nil if not found
+	Get(context.Context, string) ([]byte, error)
+
+	// Set will store data. The data can be retrieved by the same
+	// component after a process restart, using the same key
+	Set(context.Context, string, []byte) error
+
+	// Delete will delete data associated with the specified key
+	Delete(context.Context, string) error
+}