resolves #19403 Report the rust compiler version used in the prometheus metrics (#19643)

# What is done?
- resolves #19403
- Adds build-time Rust compiler detection and captures the rustc
--version value during the build.
- Exposes the captured compiler version from the Rust extension via a
new Python-callable function.
- Exports a new Prometheus metric for rustc version.

# How to test?
- compile `poetry install`
- add `enable_metrics: true` and 
```yaml
    resources:
    - compress: false
      names:
      - client
      - federation
      - metrics
```
to homeserver.yaml
- start synapse
- find the rustc version at `http://localhost:8008/_synapse/metrics`

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct (run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

---------

Co-authored-by: Quentin Gliech <quenting@element.io>

Author: RamReso

Cargo.lock

@@ -0,0 +1 @@
+Report the Rust compiler version used in the Prometheus metrics. Contributed by Noah Markert.

changelog.d/19643.feature

@@ -64,3 +64,4 @@ default = ["extension-module"]
 [build-dependencies]
 blake2 = "0.10.4"
 hex = "0.4.3"
+rustc_version = "0.4.1"

rust/Cargo.toml

@@ -8,6 +8,7 @@
 use std::path::PathBuf;
 
 use blake2::{Blake2b512, Digest};
+use rustc_version::version_meta;
 
 fn main() -> Result<(), std::io::Error> {
     let mut dirs = vec![PathBuf::from("src")];
@@ -48,6 +49,11 @@ fn main() -> Result<(), std::io::Error> {
     let hex_digest = hex::encode(hasher.finalize());
     println!("cargo:rustc-env=SYNAPSE_RUST_DIGEST={hex_digest}");
 
+    let rustc_version = version_meta()
+        .map(|v| v.short_version_string)
+        .unwrap_or_else(|_| "unknown".to_string());
+    println!("cargo:rustc-env=SYNAPSE_RUSTC_VERSION={}", rustc_version,);
+
     // The default rules don't pick up trivial changes to the workspace config
     // files, but we need to rebuild if those change to pick up the changed
     // hashes.

rust/build.rs

@@ -30,6 +30,14 @@ fn get_rust_file_digest() -> &'static str {
     env!("SYNAPSE_RUST_DIGEST")
 }
 
+/// Returns the `rustc` version used when this native module was built.
+///
+/// This value is embedded at build time, so it can be exported as a prometheus metrics.
+#[pyfunction]
+pub fn get_rustc_version() -> &'static str {
+    env!("SYNAPSE_RUSTC_VERSION")
+}
+
 /// Formats the sum of two numbers as string.
 #[pyfunction]
 #[pyo3(text_signature = "(a, b, /)")]
@@ -50,6 +58,7 @@ fn reset_logging_config() {
 fn synapse_rust(py: Python<'_>, m: &Bound<'_, PyModule>) -> PyResult<()> {
     m.add_function(wrap_pyfunction!(sum_as_string, m)?)?;
     m.add_function(wrap_pyfunction!(get_rust_file_digest, m)?)?;
+    m.add_function(wrap_pyfunction!(get_rustc_version, m)?)?;
     m.add_function(wrap_pyfunction!(reset_logging_config, m)?)?;
 
     acl::register_module(py, m)?;

rust/src/lib.rs

@@ -62,13 +62,17 @@
 import synapse.metrics._reactor_metrics  # noqa: F401
 from synapse.metrics._gc import MIN_TIME_BETWEEN_GCS, install_gc_manager
 from synapse.metrics._types import Collector
+from synapse.synapse_rust import get_rustc_version
 from synapse.types import StrSequence
 from synapse.util import SYNAPSE_VERSION
 
 logger = logging.getLogger(__name__)
 
 METRICS_PREFIX = "/_synapse/metrics"
 
+# Rust version used for compilation
+RUSTC_VERSION = get_rustc_version()
+
 HAVE_PROC_SELF_STAT = os.path.exists("/proc/self/stat")
 
 SERVER_NAME_LABEL = "server_name"
@@ -672,15 +676,17 @@ def collect(self) -> Iterable[Metric]:
 # consider this process-level because all Synapse homeservers running in the process
 # will use the same Synapse version.
 build_info = Gauge(  # type: ignore[missing-server-name-label]
-    "synapse_build_info", "Build information", ["pythonversion", "version", "osversion"]
+    "synapse_build_info",
+    "Build information",
+    ["pythonversion", "version", "osversion", "rustcversion"],
 )
 build_info.labels(
     " ".join([platform.python_implementation(), platform.python_version()]),
     SYNAPSE_VERSION,
     " ".join([platform.system(), platform.release()]),
+    RUSTC_VERSION,
 ).set(1)
 
-
 synapse_server_name_info = Gauge(
     "synapse_server_name_info",
     "Maps Synapse `server_name`s to the `instance`s they're hosted on",

synapse/metrics/__init__.py

@@ -1,3 +1,4 @@
 def sum_as_string(a: int, b: int) -> str: ...
 def get_rust_file_digest() -> str: ...
 def reset_logging_config() -> None: ...
+def get_rustc_version() -> str: ...