[receiver/systemd] Promote systemd receiver to alpha (#44647)

#### Description
This promotes the systemd receiver to alpha stability. While this is by
no means feature complete (e.g. per-unit CPU and memory usage, unit
uptime), its current functionality is still useful (for observability
around unit failures), and it's clear there is demand for it (e.g.
#44420).

There are still some open questions about the shape of the exported
metrics, such as whether to represent each unit as a resource or not (as
discussed in #33532, and what units to default to (see #44646), but I
think we'll be better able to answer those once this is used in the
wild.

#### Documentation
Additional examples have been added to the receiver's README.

Author: SquidDev

.chloggen/systemd-alpha.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. receiver/filelog)
+component: receiver/systemd
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Promote systemd receiver to alpha
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [33532]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

receiver/systemdreceiver/README.md

@@ -3,15 +3,16 @@
 <!-- status autogenerated section -->
 | Status        |           |
 | ------------- |-----------|
-| Stability     | [development]: metrics   |
+| Stability     | [alpha]: metrics   |
 | Unsupported Platforms | darwin, windows |
-| Distributions | [] |
+| Distributions | [contrib] |
 | Issues        | [![Open issues](https://img.shields.io/github/issues-search/open-telemetry/opentelemetry-collector-contrib?query=is%3Aissue%20is%3Aopen%20label%3Areceiver%2Fsystemd%20&label=open&color=orange&logo=opentelemetry)](https://github.com/open-telemetry/opentelemetry-collector-contrib/issues?q=is%3Aopen+is%3Aissue+label%3Areceiver%2Fsystemd) [![Closed issues](https://img.shields.io/github/issues-search/open-telemetry/opentelemetry-collector-contrib?query=is%3Aissue%20is%3Aclosed%20label%3Areceiver%2Fsystemd%20&label=closed&color=blue&logo=opentelemetry)](https://github.com/open-telemetry/opentelemetry-collector-contrib/issues?q=is%3Aclosed+is%3Aissue+label%3Areceiver%2Fsystemd) |
 | Code coverage | [![codecov](https://codecov.io/github/open-telemetry/opentelemetry-collector-contrib/graph/main/badge.svg?component=receiver_systemd)](https://app.codecov.io/gh/open-telemetry/opentelemetry-collector-contrib/tree/main/?components%5B0%5D=receiver_systemd&displayType=list) |
 | [Code Owners](https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/CONTRIBUTING.md#becoming-a-code-owner)    | [@atoulme](https://www.github.com/atoulme) |
 | Emeritus      | [@Hemansh31](https://www.github.com/Hemansh31) |
 
-[development]: https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/component-stability.md#development
+[alpha]: https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/component-stability.md#alpha
+[contrib]: https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol-contrib
 <!-- end autogenerated section -->
 
 The systemd receiver gathers metrics for locally running systemd units.
@@ -38,3 +39,21 @@ systemd.unit.state{systemd.unit.name="nginx", systemd.unit.active_state="refresh
 | `units` | `["*.service"]`  | The units to scrape, as a list of [patterns].                        |
 
 [patterns]: https://www.freedesktop.org/software/systemd/man/latest/systemctl.html#Parameter%20Syntax
+
+## Example
+### Basic configuration
+In its default configuration, the systemd receiver will scrape all system-level services:
+
+```yaml
+  receivers:
+    systemd:
+```
+
+### Advanced configuration
+
+```yaml
+  receivers:
+    systemd:
+        scope: user
+        units: ["emacs.service"]
+```

receiver/systemdreceiver/documentation.md

@@ -18,7 +18,7 @@ metrics:
 
 | Unit | Metric Type | Value Type | Aggregation Temporality | Monotonic | Stability |
 | ---- | ----------- | ---------- | ----------------------- | --------- | --------- |
-| 1 | Sum | Int | Cumulative | false | Development |
+| 1 | Sum | Int | Cumulative | false | Alpha |
 
 #### Attributes
 

receiver/systemdreceiver/internal/metadata/generated_status.go

@@ -3,8 +3,8 @@ type: systemd
 status:
   class: receiver
   stability:
-    development: [metrics]
-  distributions: []
+    alpha: [metrics]
+  distributions: [contrib]
   codeowners:
     active: [atoulme]
     emeritus: [Hemansh31]
@@ -35,7 +35,7 @@ metrics:
     description: 1 if the check resulted in active_state matching the current state, otherwise 0.
     enabled: true
     stability:
-      level: development
+      level: alpha
     sum:
       value_type: int
       aggregation_temporality: cumulative

receiver/systemdreceiver/metadata.yaml

@@ -221,6 +221,7 @@ components:
         - statsd
         - stef
         - syslog
+        - systemd
         - tcpcheck
         - tcplog
         - tlscheck