milestone 26: upstream health answers for the selected period

src/web/server.zig

@@ -39,6 +39,7 @@ const logger_mod = @import("../storage/logger.zig");
 const manager_mod = @import("../filter/manager.zig");
 const model = @import("../config/model.zig");
 const pause_mod = @import("../server/pause.zig");
+const history_mod = @import("../upstream/history.zig");
 const pool_mod = @import("../upstream/pool.zig");
 const query_sink = @import("../server/query_sink.zig");
 const retention_mod = @import("../storage/retention.zig");
@@ -135,6 +136,10 @@ pub const WebState = struct {
     client_names: ?*client_names.Resolver = null,
     manager: ?*manager_mod.Manager = null,
     pool: ?*pool_mod.Pool = null,
+    /// The upstream-outcome accumulator, for `metrics.collect` and the
+    /// `/api/health` rollup (m26 ruling 7). The ranged endpoint reads the
+    /// flushed rows through `querylog_db`, not through this.
+    history: ?*history_mod.Accumulator = null,
     monitor: ?*disk_monitor.Monitor = null,
     /// The local records and forward zones the DNS path reads. The
     /// local-records and forward-zones handlers rebuild and swap them