Ensure ordered DRA artifacts (#39270)

As things are now we allow parallel builds on the
packaging pipeline, which could result in out of order
artifacts (depending on which one takes longer to finish).

This commit implements two "queues" (snapshot/staging)
to ensure ordered builds of DRA artifacts.

Relates elastic/ingest-dev#3095

.buildkite/packaging.pipeline.yml

@@ -12,11 +12,32 @@ env:
   PLATFORMS_ARM: "linux/arm64"
 
 steps:
+  # we use concurrency gates (https://buildkite.com/blog/concurrency-gates)
+  # to implement two FIFO queues for DRA-snapshot and DRA-staging
+  # this prevents parallel builds and possibility of publishing out of order DRA artifacts if the first job takes longer than the second
+
+  - name: Start of concurrency group for DRA Snapshot
+    if: build.branch =~ /^\d+\.\d+$$/ || build.branch == 'main' || build.env('RUN_SNAPSHOT') == "true"
+    command: echo "--> Start of concurrency gate dra-snapshot"
+    concurrency_group: "dra-gate-snapshot"
+    concurrency: 1
+    key: start-gate-snapshot
+
+  - name: Start of concurrency group for DRA Staging
+    if: build.branch =~ /^\d+\.\d+$$/
+    command: echo "--> Start of concurrency gate dra-staging"
+    concurrency_group: "dra-gate-staging"
+    concurrency: 1
+    key: start-gate-staging
+
+  - wait
+
   - group: Beats dashboards
     key: dashboards
     steps:
       - label: Snapshot dashboards
         if: build.branch =~ /^\d+\.\d+$$/ || build.branch == 'main' || build.env('RUN_SNAPSHOT') == "true"
+        depends_on: start-gate-snapshot
         key: dashboards-snapshot
         # TODO: container with go and make
         agents:
@@ -34,6 +55,7 @@ steps:
 
       - label: Staging dashboards
         if: build.branch =~ /^\d+\.\d+$$/
+        depends_on: start-gate-staging
         key: dashboards-staging
         # TODO: container with go and make
         agents:
@@ -52,6 +74,7 @@ steps:
   - group: Packaging snapshot
     if: build.branch =~ /^\d+\.\d+$$/ || build.branch == 'main' || build.env('RUN_SNAPSHOT') == "true"
     key: packaging-snapshot
+    depends_on: start-gate-snapshot
     steps:
       - label: "SNAPSHOT: {{matrix}}"
         env:
@@ -111,6 +134,7 @@ steps:
   - group: Packaging staging
 
     key: packaging-staging
+    depends_on: start-gate-staging
     ## Only for release
     if: build.branch =~ /^\d+\.\d+$$/
     steps:
@@ -179,6 +203,7 @@ steps:
         env:
           DRA_WORKFLOW: snapshot
         depends_on:
+          - start-gate-snapshot
           - packaging-snapshot
           - dashboards-snapshot
         command: |
@@ -197,6 +222,7 @@ steps:
         env:
           DRA_WORKFLOW: staging
         depends_on:
+          - start-gate-staging
           - packaging-staging
           - dashboards-staging
         command: |
@@ -207,3 +233,17 @@ steps:
           provider: gcp
           image: "${IMAGE_UBUNTU_X86_64}"
           machineType: "${GCP_DEFAULT_MACHINE_TYPE}"
+
+  - wait
+
+  - command: echo "End of concurrency gate dra-snapshot <--"
+    if: build.branch =~ /^\d+\.\d+$$/ || build.branch == 'main' || build.env('RUN_SNAPSHOT') == "true"
+    concurrency_group: "dra-gate-snapshot"
+    concurrency: 1
+    key: end-gate-snapshot
+
+  - command: echo "End of concurrency gate dra-staging <--"
+    if: build.branch =~ /^\d+\.\d+$$/
+    concurrency_group: "dra-gate-staging"
+    concurrency: 1
+    key: end-gate-staging