feat(python): Document default span attrs in ASGI, update for 3.0 (#14368)

Document what span attributes will be attached to transactions by
default by our builtin integrations. This PR updates ASGI. Rest will
follow.

I created a new 3.x page for this since it's new behavior in 3.0.

---------

Co-authored-by: Daniel Szoke <[email protected]>

Author: sentrivana

docs/platforms/python/integrations/asgi/index__v3.x.mdx

@@ -0,0 +1,133 @@
+---
+title: ASGI
+description: "Learn about the ASGI integration and how it adds support for ASGI applications."
+---
+
+The ASGI middleware can be used to instrument any [ASGI](https://asgi.readthedocs.io/en/latest/)-compatible web framework to attach request data for your events.
+
+<Alert>
+
+Please check our list of [supported integrations](/platforms/python/integrations/) as there might already be a specific integration (like [FastAPI](/platforms/python/integrations/fastapi/) or [Sanic](/platforms/python/integrations/sanic/)) that is easier to use and captures more useful information than our generic ASGI middleware. If that's the case, you should use the specific integration instead of this middleware.
+
+</Alert>
+
+## Install
+
+```bash {tabTitle:pip}
+pip install "sentry-sdk"
+```
+```bash {tabTitle:uv}
+uv add "sentry-sdk"
+```
+
+## Configure
+
+In addition to capturing errors, you can monitor interactions between multiple services or applications by [enabling tracing](/concepts/key-terms/tracing/). You can also collect and analyze performance profiles from real users with [profiling](/product/explore/profiling/).
+
+Select which Sentry features you'd like to install in addition to Error Monitoring to get the corresponding installation and configuration instructions below.
+
+<OnboardingOptionButtons
+  options={[
+    'error-monitoring',
+    'performance',
+    'profiling',
+  ]}
+/>
+
+```python
+import sentry_sdk
+from sentry_sdk.integrations.asgi import SentryAsgiMiddleware
+
+from my_asgi_app import app
+
+sentry_sdk.init(
+    dsn="___PUBLIC_DSN___",
+    # Add data like request headers and IP for users, if applicable;
+    # see https://docs.sentry.io/platforms/python/data-management/data-collected/ for more info
+    send_default_pii=True,
+    # ___PRODUCT_OPTION_START___ performance
+    # Set traces_sample_rate to 1.0 to capture 100%
+    # of transactions for tracing.
+    traces_sample_rate=1.0,
+    # ___PRODUCT_OPTION_END___ performance
+    # ___PRODUCT_OPTION_START___ profiling
+    # To collect profiles for all profile sessions,
+    # set `profile_session_sample_rate` to 1.0.
+    profile_session_sample_rate=1.0,
+    # Profiles will be automatically collected while
+    # there is an active span.
+    profile_lifecycle="trace",
+    # ___PRODUCT_OPTION_END___ profiling
+)
+
+app = SentryAsgiMiddleware(app)
+```
+
+The middleware supports both ASGI 2 and ASGI 3.
+
+## Verify
+
+Trigger an error in your code and see it show up in [sentry.io](https://sentry.io).
+
+```python
+sentry_sdk.init(...) # same as above
+
+def app(scope):
+    async def get_body():
+        return f"The number is: {1/0}" # raises an error!
+
+    async def asgi(receive, send):
+        await send(
+            {
+                "type": "http.response.start",
+                "status": 200,
+                "headers": [[b"content-type", b"text/plain"]],
+            }
+        )
+        await send({"type": "http.response.body", "body": await get_body()})
+
+    return asgi
+
+app = SentryAsgiMiddleware(app)
+```
+
+Run your ASGI app with unicorn:
+
+```bash
+uvicorn main:app --port 8000
+```
+
+Point your browser to [http://localhost:8000](http://localhost:8000) to trigger the error which is then sent to Sentry.
+
+Additionally, a transaction will show up in the "Performance" section on [sentry.io](https://sentry.io).
+
+## Behavior
+
+- Request data is attached to all events: **HTTP method, URL, headers**. Sentry excludes raw bodies and multipart file uploads. Sentry also excludes personally identifiable information (such as user ids, usernames, cookies, authorization headers, IP addresses) unless you set `send_default_pii` to `True`.
+
+- Each request has a separate scope. Changes to the scope within a view, for example setting a tag, will only apply to events sent as part of the request being handled.
+
+- The ASGI middleware does not behave like a regular integration. It is not initialized through an extra parameter to `init` and is not attached to a client. When capturing or supplementing events, it just uses the currently active scopes.
+
+### Default Span Attributes
+
+A set of predefined span attributes will be attached to ASGI transactions by default. These can also be used for sampling since they will also be accessible via the `sampling_context` dictionary in the [`traces_sampler`](/platforms/python/configuration/options/#traces_sampler).
+
+    | Span Attribute                  | Description                                              |
+    | ------------------------------- | -------------------------------------------------------- |
+    | `network.protocol.name`         | `type` on ASGI scope                                     |
+    | `url.scheme`                    | `scheme` on ASGI scope                                   |
+    | `url.path`                      | `path` on ASGI scope                                     |
+    | `url.query`                     | `query` on ASGI scope                                    |
+    | `network.protocol.version`      | `http_version` on ASGI scope                             |
+    | `http.request.method`           | `method` on ASGI scope                                   |
+    | `server.address`, `server.port` | `server` on ASGI scope                                   |
+    | `client.address`, `client.port` | `client` on ASGI scope                                   |
+    | `url.full`                      | full URL, reconstructed from individual ASGI scope parts |
+    | `http.request.header.{header}`  | `headers` on ASGI scope                                  |
+
+These attributes will also be sent to Sentry. If you don't want that, you can filter them out using a custom [`before_send`](/platforms/python/configuration/options/#before_send) function.
+
+## Supported Versions
+
+- Python: 3.7+