chore(docs): mark DD_RUNTIME_METRICS_INTERVAL for removal in 4.0 (#13153)

Adds a deprecation warning for DD_RUNTIME_METRICS_INTERVAL and marks it for removal in 4.0.

## Checklist
- [X] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

---------

Co-authored-by: Zachary Groves <[email protected]>

Author: vparfait

ddtrace/internal/runtime/runtime_metrics.py

@@ -8,6 +8,7 @@
 from ddtrace.internal import atexit
 from ddtrace.internal import forksafe
 from ddtrace.internal.constants import EXPERIMENTAL_FEATURES
+from ddtrace.vendor.debtcollector import deprecate
 from ddtrace.vendor.dogstatsd import DogStatsd
 
 from .. import periodic
@@ -68,6 +69,11 @@ class RuntimeMetrics(RuntimeCollectorsIterable):
 
 
 def _get_interval_or_default():
+    if "DD_RUNTIME_METRICS_INTERVAL" in os.environ:
+        deprecate(
+            "`DD_RUNTIME_METRICS_INTERVAL` is deprecated and will be removed in a future version.",
+            removal_version="4.0.0",
+        )
     return float(os.getenv("DD_RUNTIME_METRICS_INTERVAL", default=10))
 
 

releasenotes/notes/deprecate-runtime-interval-5e3b3f19d4d53b25.yaml

@@ -0,0 +1,4 @@
+---
+deprecations:
+  - |
+    ``DD_RUNTIME_METRICS_INTERVAL`` is deprecated and will be removed in the 4.0.0 release.