refactor(telemetry): memoize get_host_info via @CallOnce (#17669)

## Summary

Split out of #17667. Replace the `_host_info` module-level global + `None`-check in `ddtrace/internal/telemetry/data.py` with `@callonce`, the idiomatic no-arg memoization decorator in this codebase (already used by `ddtrace/internal/packages.py` and `ddtrace/internal/debug.py`).

- Removes the unsynchronized `None`-check race. `@callonce` also has no explicit lock, but `get_host_info` is deterministic and idempotent, so a benign double-compute on first-call contention produces identical results with no torn state.
- `get_host_info()` is still the public entry point and still returns the same dict shape.
- Originally this PR used `@cached()` (i.e. `functools.lru_cache`), but `@cached()` requires at least one hashable argument and forced a dummy `_: tuple = ()` parameter plus a wrapper function. `@callonce` is purpose-built for zero-arg functions and can be applied directly to `get_host_info`, eliminating the indirection.

## Performance

Microbenchmark on the warm path (A/B reproduction of the two implementations, 10 000 calls per sample, 11 samples):

| Per-call (warm) | Before (global None-check) | After (`@callonce`) | Δ |
|---|---:|---:|---:|
| Best | ~20 ns | ~30-60 ns | negligible |

`get_host_info()` is called at most once per telemetry heartbeat (~60 s), so any small constant-factor difference is not observable in practice. The motivation is code clarity and consistency with the rest of the codebase, not speed.

## Test plan

- [x] `tests/telemetry/test_data.py::test_get_host_info` passes on Python 3.14 via `scripts/run-tests`.
- [x] Existing `tests/telemetry/` suite passes (no behavior change at the function boundary — same dict contents, same call signature).
- [x] No release note needed — internal refactor, `changelog/no-changelog` applied.

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Co-authored-by: alberto.vara <alberto.vara@datadoghq.com>

Author: avara1986

ddtrace/internal/telemetry/data.py

@@ -6,6 +6,7 @@
 from ddtrace.internal.constants import DEFAULT_SERVICE_NAME
 from ddtrace.internal.runtime.container import get_container_info
 from ddtrace.internal.utils.cache import cached
+from ddtrace.internal.utils.cache import callonce
 from ddtrace.version import __version__
 
 from ..hostname import get_hostname
@@ -77,23 +78,18 @@ def get_application(service: str, version: str, env: str) -> dict:
     return _get_application((service, version, env))
 
 
-_host_info = None
-
-
+@callonce
 def get_host_info() -> dict:
-    """Creates a dictionary to store host data using the platform module"""
-    global _host_info
-    if _host_info is None:
-        _host_info = {
-            "os": platform.system(),
-            "hostname": get_hostname(),
-            "os_version": _get_os_version(),
-            "kernel_name": platform.system(),
-            "kernel_release": platform.release(),
-            "kernel_version": platform.version(),
-            "container_id": _get_container_id(),
-        }
-    return _host_info
+    """Creates a dictionary to store host data using the platform module."""
+    return {
+        "os": platform.system(),
+        "hostname": get_hostname(),
+        "os_version": _get_os_version(),
+        "kernel_name": platform.system(),
+        "kernel_release": platform.release(),
+        "kernel_version": platform.version(),
+        "container_id": _get_container_id(),
+    }
 
 
 def _get_sysconfig_var(key: str) -> str: