feat(sentry): Add tags to sentry configuration (#3944)

We have many regions for a given environment, when an issue arise it is
impossible for us to know in which region the issue occurred as the only
tag included in the sentry issue is the environment one.

This adds configurable tags in sentry configuration. Each event sent by
Sentry relay will have those tags appended.

Fix #3842

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+**Features:**
+
+- Add a config option to add default tags to all Relay Sentry events. ([#3944](https://github.com/getsentry/relay/pull/3944))
+
 ## 24.9.0
 
 **Bug Fixes**:

relay-log/src/setup.rs

@@ -1,8 +1,10 @@
 use std::borrow::Cow;
+use std::collections::BTreeMap;
 use std::env;
 use std::fmt::{self, Display};
 use std::path::PathBuf;
 use std::str::FromStr;
+use std::sync::Arc;
 
 use relay_common::impl_str_serde;
 use sentry::types::Dsn;
@@ -171,6 +173,9 @@ pub struct SentryConfig {
     /// Sets the environment for this service.
     pub environment: Option<Cow<'static, str>>,
 
+    /// Add defaults tags to the events emitted by Relay
+    pub default_tags: Option<BTreeMap<String, String>>,
+
     /// Internal. Enables crash handling and sets the absolute path to where minidumps should be
     /// cached on disk. The path is created if it doesn't exist. Path must be UTF-8.
     pub _crash_db: Option<PathBuf>,
@@ -191,6 +196,7 @@ impl Default for SentryConfig {
                 .ok(),
             enabled: false,
             environment: None,
+            default_tags: None,
             _crash_db: None,
         }
     }
@@ -281,15 +287,29 @@ pub fn init(config: &LogConfig, sentry: &SentryConfig) {
         .init();
 
     if let Some(dsn) = sentry.enabled_dsn() {
-        let guard = sentry::init(sentry::ClientOptions {
+        let mut options = sentry::ClientOptions {
             dsn: Some(dsn).cloned(),
             in_app_include: vec!["relay"],
             release: Some(RELEASE.into()),
             attach_stacktrace: config.enable_backtraces,
             environment: sentry.environment.clone(),
             traces_sample_rate: config.traces_sample_rate,
             ..Default::default()
-        });
+        };
+
+        // If `default_tags` is set in Sentry configuration install the `before_send` hook
+        // in order to inject said tags into each event
+        if let Some(default_tags) = sentry.default_tags.clone() {
+            // Install hook
+            options.before_send = Some(Arc::new(move |mut event| {
+                // Extend `event.tags` with `default_tags` without replacing tags already present
+                let previous_event_tags = std::mem::replace(&mut event.tags, default_tags.clone());
+                event.tags.extend(previous_event_tags);
+                Some(event)
+            }));
+        }
+
+        let guard = sentry::init(options);
 
         // Keep the client initialized. The client is flushed manually in `main`.
         std::mem::forget(guard);