Move metricstarttimeprocessor to beta (#43912)

#### Description

Move the metricstarttime processor to beta. We have been using it in
production, and have fixed issues that have come up. At this point, we
don't see needing any breaking chagnes, so it is time to move it to
beta, and replace the adjustment in the prometheus receiver.

#### Link to tracking issue
Part of
#43656

@ridwanmsharif

Author: dashpole

.chloggen/metricstarttime_beta.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: processor/metricstarttime
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Graduate the metricstarttimeprocessor to beta.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [43656]
+
+# (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: []

processor/metricstarttimeprocessor/README.md

@@ -1,13 +1,13 @@
 <!-- status autogenerated section -->
 | Status        |           |
 | ------------- |-----------|
-| Stability     | [alpha]: metrics   |
+| Stability     | [beta]: metrics   |
 | Distributions | [contrib] |
 | Issues        | [![Open issues](https://img.shields.io/github/issues-search/open-telemetry/opentelemetry-collector-contrib?query=is%3Aissue%20is%3Aopen%20label%3Aprocessor%2Fmetricstarttime%20&label=open&color=orange&logo=opentelemetry)](https://github.com/open-telemetry/opentelemetry-collector-contrib/issues?q=is%3Aopen+is%3Aissue+label%3Aprocessor%2Fmetricstarttime) [![Closed issues](https://img.shields.io/github/issues-search/open-telemetry/opentelemetry-collector-contrib?query=is%3Aissue%20is%3Aclosed%20label%3Aprocessor%2Fmetricstarttime%20&label=closed&color=blue&logo=opentelemetry)](https://github.com/open-telemetry/opentelemetry-collector-contrib/issues?q=is%3Aclosed+is%3Aissue+label%3Aprocessor%2Fmetricstarttime) |
 | Code coverage | [![codecov](https://codecov.io/github/open-telemetry/opentelemetry-collector-contrib/graph/main/badge.svg?component=processor_metricstarttime)](https://app.codecov.io/gh/open-telemetry/opentelemetry-collector-contrib/tree/main/?components%5B0%5D=processor_metricstarttime&displayType=list) |
 | [Code Owners](https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/CONTRIBUTING.md#becoming-a-code-owner)    | [@dashpole](https://www.github.com/dashpole), [@ridwanmsharif](https://www.github.com/ridwanmsharif) |
 
-[alpha]: https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/component-stability.md#alpha
+[beta]: https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/component-stability.md#beta
 [contrib]: https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol-contrib
 <!-- end autogenerated section -->
 

processor/metricstarttimeprocessor/internal/metadata/generated_status.go

@@ -3,7 +3,7 @@ type: metricstarttime
 status:
   class: processor
   stability:
-    alpha: [metrics]
+    beta: [metrics]
   distributions: [contrib]
   codeowners:
     active: [dashpole, ridwanmsharif]