chore(tornado): v4.0 deprecations (#15274)

## Description

Deprecated Tornado versions older than v6.1 and programmatic
configuration via ddtrace.contrib.tornado. Users should upgrade to
Tornado v6.1+ and use environment variables with import ddtrace.auto.

## Motivation

Tornado v6.1 (released in 2020) added contextvars support, eliminating
the need for a custom Tornado Context Provider. This allows us to remove
the last integration specific Context Provider and simplify our context
API.

---------

Co-authored-by: Brett Langdon <brett.langdon@datadoghq.com>

Author: mabdinur

ddtrace/contrib/internal/tornado/patch.py

@@ -7,8 +7,10 @@
 import ddtrace
 from ddtrace import config
 from ddtrace.contrib.internal.tornado.stack_context import context_provider
+from ddtrace.internal.utils.deprecations import DDTraceDeprecationWarning
 from ddtrace.internal.utils.formats import asbool
 from ddtrace.internal.utils.wrappers import unwrap as _u
+from ddtrace.vendor.debtcollector import deprecate
 
 from . import application
 from . import decorators
@@ -26,7 +28,19 @@
 
 def get_version():
     # type: () -> str
-    return getattr(tornado, "version", "")
+    return getattr(tornado, "version", "0.0.0")
+
+
+VERSION_TUPLE = tuple([int(x) for x in get_version().split(".")])
+
+if VERSION_TUPLE < (6, 1, 0):
+    deprecate(
+        f"Tornado {VERSION_TUPLE} is deprecated",
+        message="Use Tornado v6.1 or later and configure tracing using "
+        "environment variables and ``ddtrace-run`` or ``import ddtrace.auto`` instead.",
+        category=DDTraceDeprecationWarning,
+        removal_version="4.0.0",
+    )
 
 
 def _supported_versions() -> Dict[str, str]:

ddtrace/contrib/tornado.py

@@ -100,8 +100,17 @@ def log_exception(self, typ, value, tb):
 from ddtrace.contrib.internal.tornado.stack_context import TracerStackContext
 from ddtrace.contrib.internal.tornado.stack_context import context_provider
 from ddtrace.contrib.internal.tornado.stack_context import run_with_trace_context
+from ddtrace.internal.utils.deprecations import DDTraceDeprecationWarning
+from ddtrace.vendor.debtcollector import deprecate
 
 
+deprecate(
+    "ddtrace.contrib.tornado is deprecated",
+    message="Use ``import ddtrace.auto`` and ``DD_PATCH_MODULES`` to configure tracing for Tornado.",
+    category=DDTraceDeprecationWarning,
+    removal_version="4.0.0",
+)
+
 __all__ = [
     "context_provider",
     "run_with_trace_context",

releasenotes/notes/deprecate-older-tornado-versions-381d2f4e6c4c8288.yaml

@@ -0,0 +1,4 @@
+---
+deprecations:
+  - |
+    tornado: Deprecated support for Tornado versions older than v6.1. Use Tornado v6.1 or later.

releasenotes/notes/deprecate-tornado-programmatic-api-5f7a8b9c1d2e3f4a.yaml

@@ -0,0 +1,4 @@
+---
+deprecations:
+  - |
+    tornado: Deprecates programmatic tracing configuration via the ``ddtrace.contrib.tornado`` module. Configure tracing using environment variables and ``import ddtrace.auto`` instead.