otel: say which of processes/process is configured

They differ by one letter and a grep for the shorter name matches both.
The summary is what ships; the per-process scraper is deliberately absent,
and a reader (human or grep) cannot tell from the diff alone.

nix/host-modules/otel.nix

@@ -349,6 +349,57 @@ in
         settings = {
           receivers = {
             otlp.protocols.http.endpoint = listen;
+
+            # The host itself, which nothing else measures. hive-c0re emits
+            # per-container metrics and each service exports its own, so the
+            # store could answer "is this container using more memory than
+            # last week" and could not answer "is the machine under them out
+            # of memory" — the question every one of those readings is
+            # implicitly relative to.
+            #
+            # Here rather than in ./swarm-otel.nix because a host is owned by
+            # the hive running on it: the swarm tier runs one collector for
+            # the whole swarm and would have to reach *other* machines to see
+            # theirs. Consequence worth knowing before reading a dashboard:
+            # these samples carry `hive` and no `agent`, so "which host" is
+            # answered by "which hive" — true only while a hive is one host.
+            #
+            # Every scraper the pinned receiver offers except `process`, and
+            # the exception is not squeamishness about volume: `process` adds
+            # a series set PER RUNNING PROCESS, which on a hive host means
+            # every agent's claude and every nix build. That is unbounded and
+            # churning, where the nine below are a fixed handful per host
+            # forever. Coverage and cardinality are different axes and only
+            # the second one can hurt the store.
+            #
+            # Names verified against THIS collector version rather than
+            # upstream's docs, with a deliberately bogus scraper rejected in
+            # the same run so that "accepted" is distinguishable from "the
+            # validator ignores unknown keys". `validateConfigFile` above
+            # keeps that honest at build time.
+            hostmetrics = {
+              # Stated rather than inherited: the upstream default is a
+              # value that can change under a version bump, and the sample
+              # interval is the one knob here with a storage bill attached.
+              collection_interval = "60s";
+              scrapers = {
+                cpu = { };
+                memory = { };
+                filesystem = { };
+                load = { };
+                disk = { };
+                network = { };
+                paging = { };
+                # ⚠️ `processes` is the SUMMARY -- how many are running,
+                # blocked, sleeping. A handful of series per host. Its
+                # singular sibling `process` is the per-process one and is
+                # deliberately absent above; the two differ by one letter and
+                # a grep for the shorter name matches both, so read the
+                # trailing `s` before concluding which is configured here.
+                processes = { };
+                system = { };
+              };
+            };
           }
           # Only when a service has actually declared a target. An enabled
           # `prometheus` receiver with an empty `scrape_configs` renders,
@@ -407,7 +458,15 @@ in
             # `optional` matters too: a pipeline naming a receiver that no
             # longer exists is a startup failure, which `validateConfigFile`
             # below turns into a build failure rather than a broken deploy.
-            receivers = [ "otlp" ] ++ lib.optional (otel.scrapeTargets != { }) "prometheus";
+            # `hostmetrics` is unconditional, unlike `prometheus` above: it
+            # has no per-hive configuration to be empty of, so there is no
+            # inert shape to guard against — the receiver either collects the
+            # host's metrics or the host has none collected at all.
+            receivers = [
+              "otlp"
+              "hostmetrics"
+            ]
+            ++ lib.optional (otel.scrapeTargets != { }) "prometheus";
             exporters = [ swarmName ];
           };
         }