m/pkg/event/etcd: implement etcd-backed Value

This implements Event Value stored in etcd, with each Value
corresponding to a single KV value stored in etcd.

Comes with more lines of unit tests than lines of code.

Change-Id: I5514f211ded6640836ed801ddaf1b2fcc31ae552
Reviewed-on: https://review.monogon.dev/c/monogon/+/64
Reviewed-by: Lorenz Brun <lorenz@nexantic.com>
diff --git a/metropolis/pkg/event/etcd/BUILD.bazel b/metropolis/pkg/event/etcd/BUILD.bazel
new file mode 100644
index 0000000..b00bc47
--- /dev/null
+++ b/metropolis/pkg/event/etcd/BUILD.bazel
@@ -0,0 +1,28 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["etcd.go"],
+    importpath = "source.monogon.dev/metropolis/pkg/event/etcd",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//metropolis/node/core/consensus/client:go_default_library",
+        "//metropolis/pkg/event:go_default_library",
+        "@com_github_cenkalti_backoff_v4//:go_default_library",
+        "@io_etcd_go_etcd//clientv3:go_default_library",
+    ],
+)
+
+go_test(
+    name = "go_default_test",
+    srcs = ["etcd_test.go"],
+    embed = [":go_default_library"],
+    deps = [
+        "//metropolis/node/core/consensus/client:go_default_library",
+        "//metropolis/pkg/event:go_default_library",
+        "@io_etcd_go_etcd//clientv3:go_default_library",
+        "@io_etcd_go_etcd//etcdserver/api/v3rpc/rpctypes:go_default_library",
+        "@io_etcd_go_etcd//integration:go_default_library",
+        "@org_golang_google_grpc//codes:go_default_library",
+    ],
+)