swarm-otel: the operator chooses which signals go upstream

Adds services.hyperhive.otel.upstreamSignals, a list of "metrics" and/or
"logs" naming the signals the swarm collector sends to otel.endpoint. The
swarm's own stores always receive everything. The upstream exporter is
only defined when at least one signal uses it, since otelcol refuses an
exporter no pipeline references.

On deploy: nothing changes until the option is set; the default is
[ "metrics" "logs" ], today's behaviour. To stop the 404 flood from an
upstream that has no /v1/logs, set services.hyperhive.otel.upstreamSignals
= [ "metrics" ].

nix/host-modules/otel.nix

@@ -88,6 +88,32 @@ in
       '';
     };
 
+    upstreamSignals = lib.mkOption {
+      type = lib.types.listOf (
+        lib.types.enum [
+          "metrics"
+          "logs"
+        ]
+      );
+      default = [
+        "metrics"
+        "logs"
+      ];
+      example = [ "metrics" ];
+      description = ''
+        Which signals the swarm's collector sends to
+        {option}`services.hyperhive.otel.endpoint`. The swarm's own metrics
+        and log stores always receive everything, whatever this lists.
+
+        Use it when the upstream does not accept a signal, for example one
+        that answers 404 on `/v1/logs`: list only `"metrics"` and the logs
+        pipelines stop exporting upstream. An empty list defines no upstream
+        exporter at all. No pipeline exports traces, so they have no entry.
+
+        Has no effect while {option}`services.hyperhive.otel.endpoint` is empty.
+      '';
+    };
+
     protocol = lib.mkOption {
       type = lib.types.enum [
         "http/protobuf"