introduce LaunchTimestamp to identify process restarts

This patch adds a LaunchTimestamp type to the `metrics` crate,
along with a `libmetric_` Prometheus metric.

The initial user is pageserver.
In addition to exposing the Prometheus metric, it also reproduces
the launch timestamp as a header in the API responses.

The motivation for this is that we plan to scrape the pageserver's
/v1/tenant/:tenant_id/timeline/:timeline_id/layer
HTTP endpoint over time. It will soon expose access metrics (#3496)
which reset upon process restart. We will use the pageserver's launch
ID to identify a restart between two scrape points.

However, there are other potential uses. For example, we could use
the Prometheus metric to annotate Grafana plots whenever the launch
timestamp changes.

Cargo.lock

@@ -2019,6 +2019,7 @@ dependencies = [
 name = "metrics"
 version = "0.1.0"
 dependencies = [
+ "chrono",
  "libc",
  "once_cell",
  "prometheus",

libs/metrics/Cargo.toml

@@ -8,5 +8,6 @@ license.workspace = true
 prometheus.workspace = true
 libc.workspace = true
 once_cell.workspace = true
+chrono.workspace = true
 
 workspace_hack.workspace = true

libs/metrics/src/launch_timestamp.rs

@@ -0,0 +1,34 @@
+//! A timestamp captured at process startup to identify restarts of the process, e.g., in logs and metrics.
+
+use chrono::Utc;
+
+use super::register_uint_gauge;
+use std::fmt::Display;
+
+pub struct LaunchTimestamp(chrono::DateTime<Utc>);
+
+impl LaunchTimestamp {
+    pub fn generate() -> Self {
+        LaunchTimestamp(Utc::now())
+    }
+}
+
+impl Display for LaunchTimestamp {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+pub fn set_launch_timestamp_metric(launch_ts: &'static LaunchTimestamp) {
+    let millis_since_epoch: u64 = launch_ts
+        .0
+        .timestamp_millis()
+        .try_into()
+        .expect("we're after the epoch, this should be positive");
+    let metric = register_uint_gauge!(
+        "libmetrics_launch_timestamp",
+        "Timestamp (millis since epoch) at wich the process launched."
+    )
+    .unwrap();
+    metric.set(millis_since_epoch);
+}

libs/metrics/src/lib.rs

@@ -20,6 +20,7 @@ pub use prometheus::{register_int_gauge_vec, IntGaugeVec};
 pub use prometheus::{Encoder, TextEncoder};
 use prometheus::{Registry, Result};
 
+pub mod launch_timestamp;
 mod wrappers;
 pub use wrappers::{CountedReader, CountedWriter};
 
@@ -34,6 +35,14 @@ macro_rules! register_uint_gauge_vec {
     }};
 }
 
+#[macro_export]
+macro_rules! register_uint_gauge {
+    ($NAME:expr, $HELP:expr $(,)?) => {{
+        let gauge = $crate::UIntGauge::new($NAME, $HELP).unwrap();
+        $crate::register(Box::new(gauge.clone())).map(|_| gauge)
+    }};
+}
+
 /// Special internal registry, to collect metrics independently from the default registry.
 /// Was introduced to fix deadlock with lazy registration of metrics in the default registry.
 static INTERNAL_REGISTRY: Lazy<Registry> = Lazy::new(Registry::new);

libs/utils/src/http/endpoint.rs

@@ -1,7 +1,8 @@
 use crate::auth::{Claims, JwtAuth};
 use crate::http::error;
-use anyhow::anyhow;
-use hyper::header::AUTHORIZATION;
+use anyhow::{anyhow, Context};
+use hyper::header::{HeaderName, AUTHORIZATION};
+use hyper::http::HeaderValue;
 use hyper::{header::CONTENT_TYPE, Body, Request, Response, Server};
 use metrics::{register_int_counter, Encoder, IntCounter, TextEncoder};
 use once_cell::sync::Lazy;
@@ -13,6 +14,7 @@ use tracing::info;
 
 use std::future::Future;
 use std::net::TcpListener;
+use std::str::FromStr;
 
 use super::error::ApiError;
 
@@ -143,6 +145,38 @@ pub fn auth_middleware<B: hyper::body::HttpBody + Send + Sync + 'static>(
     })
 }
 
+pub fn add_response_header_middleware<B>(
+    header: &str,
+    value: &str,
+) -> anyhow::Result<Middleware<B, ApiError>>
+where
+    B: hyper::body::HttpBody + Send + Sync + 'static,
+{
+    let name =
+        HeaderName::from_str(header).with_context(|| format!("invalid header name: {header}"))?;
+    let value =
+        HeaderValue::from_str(value).with_context(|| format!("invalid header value: {value}"))?;
+    Ok(Middleware::post_with_info(
+        move |mut response, request_info| {
+            let name = name.clone();
+            let value = value.clone();
+            async move {
+                let headers = response.headers_mut();
+                if headers.contains_key(&name) {
+                    tracing::warn!(
+                        "{} response already contains header {:?}",
+                        request_info.uri(),
+                        &name,
+                    );
+                } else {
+                    headers.insert(name, value);
+                }
+                Ok(response)
+            }
+        },
+    ))
+}
+
 pub fn check_permission_with(
     req: &Request<Body>,
     check_permission: impl Fn(&Claims) -> Result<(), anyhow::Error>,

pageserver/src/bin/pageserver.rs

@@ -7,6 +7,7 @@ use std::{env, ops::ControlFlow, path::Path, str::FromStr};
 use anyhow::{anyhow, Context};
 use clap::{Arg, ArgAction, Command};
 use fail::FailScenario;
+use metrics::launch_timestamp::{set_launch_timestamp_metric, LaunchTimestamp};
 use remote_storage::GenericRemoteStorage;
 use tracing::*;
 
@@ -52,6 +53,8 @@ fn version() -> String {
 }
 
 fn main() -> anyhow::Result<()> {
+    let launch_ts = Box::leak(Box::new(LaunchTimestamp::generate()));
+
     let arg_matches = cli().get_matches();
 
     if arg_matches.get_flag("enabled-features") {
@@ -108,7 +111,7 @@ fn main() -> anyhow::Result<()> {
     virtual_file::init(conf.max_file_descriptors);
     page_cache::init(conf.page_cache_size);
 
-    start_pageserver(conf).context("Failed to start pageserver")?;
+    start_pageserver(launch_ts, conf).context("Failed to start pageserver")?;
 
     scenario.teardown();
     Ok(())
@@ -203,13 +206,24 @@ fn initialize_config(
     })
 }
 
-fn start_pageserver(conf: &'static PageServerConf) -> anyhow::Result<()> {
+fn start_pageserver(
+    launch_ts: &'static LaunchTimestamp,
+    conf: &'static PageServerConf,
+) -> anyhow::Result<()> {
     // Initialize logging
     logging::init(conf.log_format)?;
 
-    // Print version to the log, and expose it as a prometheus metric too.
-    info!("version: {}", version());
+    // Print version and launch timestamp to the log,
+    // and expose them as prometheus metrics.
+    // A changed version string indicates changed software.
+    // A changed launch timestamp indicates a pageserver restart.
+    info!(
+        "version: {} launch_timestamp: {}",
+        version(),
+        launch_ts.to_string()
+    );
     set_build_info_metric(GIT_VERSION);
+    set_launch_timestamp_metric(launch_ts);
 
     // If any failpoints were set from FAILPOINTS environment variable,
     // print them to the log for debugging purposes
@@ -307,7 +321,7 @@ fn start_pageserver(conf: &'static PageServerConf) -> anyhow::Result<()> {
     {
         let _rt_guard = MGMT_REQUEST_RUNTIME.enter();
 
-        let router = http::make_router(conf, auth.clone(), remote_storage)?
+        let router = http::make_router(conf, launch_ts, auth.clone(), remote_storage)?
             .build()
             .map_err(|err| anyhow!(err))?;
         let service = utils::http::RouterService::new(router).unwrap();

pageserver/src/http/routes.rs

@@ -3,6 +3,7 @@ use std::sync::Arc;
 use anyhow::{anyhow, Context, Result};
 use hyper::StatusCode;
 use hyper::{Body, Request, Response, Uri};
+use metrics::launch_timestamp::LaunchTimestamp;
 use pageserver_api::models::DownloadRemoteLayersTaskSpawnRequest;
 use remote_storage::GenericRemoteStorage;
 use tokio_util::sync::CancellationToken;
@@ -921,6 +922,7 @@ async fn handler_404(_: Request<Body>) -> Result<Response<Body>, ApiError> {
 
 pub fn make_router(
     conf: &'static PageServerConf,
+    launch_ts: &'static LaunchTimestamp,
     auth: Option<Arc<JwtAuth>>,
     remote_storage: Option<GenericRemoteStorage>,
 ) -> anyhow::Result<RouterBuilder<hyper::Body, ApiError>> {
@@ -937,6 +939,14 @@ pub fn make_router(
         }))
     }
 
+    router = router.middleware(
+        endpoint::add_response_header_middleware(
+            "PAGESERVER_LAUNCH_TIMESTAMP",
+            &launch_ts.to_string(),
+        )
+        .expect("construct launch timestamp header middleware"),
+    );
+
     macro_rules! testing_api {
         ($handler_desc:literal, $handler:path $(,)?) => {{
             #[cfg(not(feature = "testing"))]

test_runner/fixtures/metrics.py

@@ -50,6 +50,8 @@ PAGESERVER_GLOBAL_METRICS: Tuple[str, ...] = (
     "pageserver_storage_operations_seconds_global_count",
     "pageserver_storage_operations_seconds_global_sum",
     "pageserver_storage_operations_seconds_global_bucket",
+    "libmetrics_launch_timestamp",
+    "libmetrics_build_info",
 )
 
 PAGESERVER_PER_TENANT_METRICS: Tuple[str, ...] = (