[receiver/datadog] Update metrics stability level to alpha (#35536)

**Description:** <Describe what has changed.>
This PR updates the stability level of metrics support in the Datadog
receiver to alpha.

**Link to tracking Issue:** <Issue number if applicable>
#18278

**Testing:** <Describe what testing was performed and which tests were
added.>
N/A

**Documentation:** <Describe the documentation added.>
README updated

.chloggen/datadogreceiver-metrics-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. filelogreceiver)
+component: datadogreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Move receiver's metrics stability to alpha.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [18278]
+
+# (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/datadogreceiver/README.md

@@ -3,13 +3,11 @@
 <!-- status autogenerated section -->
 | Status        |           |
 | ------------- |-----------|
-| Stability     | [development]: metrics   |
-|               | [alpha]: traces   |
+| Stability     | [alpha]: traces, metrics   |
 | Distributions | [contrib] |
 | Issues        | [![Open issues](https://img.shields.io/github/issues-search/open-telemetry/opentelemetry-collector-contrib?query=is%3Aissue%20is%3Aopen%20label%3Areceiver%2Fdatadog%20&label=open&color=orange&logo=opentelemetry)](https://github.com/open-telemetry/opentelemetry-collector-contrib/issues?q=is%3Aopen+is%3Aissue+label%3Areceiver%2Fdatadog) [![Closed issues](https://img.shields.io/github/issues-search/open-telemetry/opentelemetry-collector-contrib?query=is%3Aissue%20is%3Aclosed%20label%3Areceiver%2Fdatadog%20&label=closed&color=blue&logo=opentelemetry)](https://github.com/open-telemetry/opentelemetry-collector-contrib/issues?q=is%3Aclosed+is%3Aissue+label%3Areceiver%2Fdatadog) |
 | [Code Owners](https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/CONTRIBUTING.md#becoming-a-code-owner)    | [@boostchicken](https://www.github.com/boostchicken), [@gouthamve](https://www.github.com/gouthamve), [@jpkrohling](https://www.github.com/jpkrohling), [@MovieStoreGuy](https://www.github.com/MovieStoreGuy) |
 
-[development]: https://github.com/open-telemetry/opentelemetry-collector#development
 [alpha]: https://github.com/open-telemetry/opentelemetry-collector#alpha
 [contrib]: https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol-contrib
 <!-- end autogenerated section -->

receiver/datadogreceiver/metadata.yaml

@@ -3,13 +3,12 @@ type: datadog
 status:
   class: receiver
   stability:
-    alpha: [traces]
-    development: [metrics]
+    alpha: [traces, metrics]
   distributions: [contrib]
   codeowners:
     active: [boostchicken, gouthamve, jpkrohling, MovieStoreGuy]
 
 tests:
   skip_lifecycle: true # Skip lifecycle tests since there are multiple receivers that run on the same port
   goleak:
-    skip: true # see https://github.com/cihub/seelog/issues/182
+    skip: true # see https://github.com/cihub/seelog/issues/182