Tag pushrebase Scuba with land_instance_id for MR QE terminal-bounce readout

Summary:
The merge-resolution QE needs the ITT *terminal* conflict-bounce (true-rescue) rate, test vs control, computed retry-inclusively. That terminal outcome has to be grouped per land across the Landcastle/Sandcastle client rebase-retry loop, but no existing column on `mononoke_land_service` is stable across that loop: `session_uuid`/`client_correlator` are per push-invocation, `sandcastle_nonce` is per job-type, and `changeset_id` changes on rebase.

This adds a stable per-land grouping key -- the Sandcastle land instance id -- to the pushrebase Scuba samples, mirroring how `mr_qe_arm` is stamped (D108612592). The readout then groups `mononoke_land_service` by `land_instance_id` to roll a land's attempts up to a single terminal outcome and slice by arm -- entirely within source_control-owned tables (no www logger / cross-team change).

- Landcastle (`LandcastleMrQeBuckets`) emits a `LAND_INSTANCE_ID` pushvar (the Sandcastle instance id) on every (re)push of an enrolled land; the same Sandcastle instance drives the whole `genPush` retry loop, so all attempts of a land carry the same value.
- Mononoke reads it into `PushrebaseFlags.land_instance_id` (UTF-8 decode at the bookmarks_movement boundary so `metaconfig_types` stays bytes-free) and stamps `land_instance_id` in the `do_pushrebase_bonsai` `with_mutated_scuba` rebind, so every downstream sample inherits it.

Observability only; absent/`None` for out-of-experiment traffic; no behavior change.

Reviewed By: YousefSalama, lmvasquezg

Differential Revision: D110041743

fbshipit-source-id: f02137439376731a582cc1275c1110ee0f5374d4

Author: RajivTS

eden/mononoke/features/pushrebase/src/lib.rs

@@ -333,6 +333,10 @@ pub async fn do_pushrebase_bonsai(
     // means every downstream `ctx.scuba()` clone inherits the field.
     let ctx = ctx.with_mutated_scuba(|mut scuba| {
         scuba.add("mr_qe_arm", config.merge_resolution_override.qe_arm_str());
+        // Per-land key to roll a land's attempts up to a terminal outcome.
+        if let Some(land_instance_id) = config.land_instance_id.as_deref() {
+            scuba.add("land_instance_id", land_instance_id);
+        }
         scuba
     });
     let ctx = &ctx;

eden/mononoke/metaconfig/parser/src/config.rs

@@ -1492,6 +1492,7 @@ mod test {
                         merge_resolution_excluded_path_prefixes: Default::default(),
                         pessimistic_locking_bookmarks: Vec::new(),
                         merge_resolution_override: MergeResolutionOverride::UseJk,
+                        land_instance_id: None,
                     },
                     block_merges: false,
                     emit_obsmarkers: false,

eden/mononoke/metaconfig/parser/src/convert/repo.rs

@@ -398,6 +398,7 @@ impl Convert for RawPushrebaseParams {
                     .map(BookmarkKey::new)
                     .collect::<Result<Vec<_>>>()?,
                 merge_resolution_override: MergeResolutionOverride::UseJk, // request-scoped, not loaded from config
+                land_instance_id: None, // request-scoped, not loaded from config
             },
             block_merges: self.block_merges.unwrap_or(default.block_merges),
             emit_obsmarkers: self.emit_obsmarkers.unwrap_or(default.emit_obsmarkers),

eden/mononoke/metaconfig/types/src/lib.rs

@@ -1028,8 +1028,13 @@ pub struct PushrebaseFlags {
     /// never loaded from configerator — set on a cloned `PushrebaseFlags`
     /// before the pushrebase call (mirrors `rewritedates`).
     pub merge_resolution_override: MergeResolutionOverride,
+    /// Per-request Sandcastle land instance id (`LAND_INSTANCE_ID` pushvar); stamped on Scuba to group a land's attempts. Observability only.
+    pub land_instance_id: Option<String>,
 }
 
+/// Pushvar key for the Sandcastle land instance id (see `PushrebaseFlags::land_instance_id`).
+pub const LAND_INSTANCE_ID_PUSHVAR_KEY: &str = "LAND_INSTANCE_ID";
+
 /// Per-request override for the `pushrebase_enable_merge_resolution` JustKnob.
 ///
 /// `UseJk` (the default) consults the JK as before. `ForceOn`/`ForceOff`
@@ -1164,6 +1169,7 @@ impl Default for PushrebaseFlags {
             merge_resolution_excluded_path_prefixes: PrefixTrie::new(),
             pessimistic_locking_bookmarks: Vec::new(),
             merge_resolution_override: MergeResolutionOverride::UseJk,
+            land_instance_id: None,
         }
     }
 }

eden/mononoke/repo_attributes/bookmarks/bookmarks_movement/src/pushrebase_onto.rs

@@ -16,6 +16,7 @@ use context::CoreContext;
 use futures_stats::TimedFutureExt;
 use hooks::CrossRepoPushSource;
 use hooks::HookManager;
+use metaconfig_types::LAND_INSTANCE_ID_PUSHVAR_KEY;
 use metaconfig_types::MergeResolutionOverride;
 use mononoke_types::BonsaiChangeset;
 use pushrebase_hook::PushrebaseHook;
@@ -130,6 +131,12 @@ impl<'op> PushrebaseOntoBookmarkOp<'op> {
                 .and_then(|p| p.get(MergeResolutionOverride::PUSHVAR_KEY))
                 .map(|b| b.as_ref()),
         );
+        // Per-land key (same on every retry push) for the terminal-bounce readout.
+        flags.land_instance_id = self
+            .pushvars
+            .and_then(|p| p.get(LAND_INSTANCE_ID_PUSHVAR_KEY))
+            .and_then(|b| std::str::from_utf8(b.as_ref()).ok())
+            .map(str::to_owned);
 
         ctx.scuba()
             .clone()