test(cosmic-swingset): Add test coverage for Prometheus output (#10954)

Extends #10941
Fixes #10911

## Description
Extracts a testable `launchChain` unit in packages/cosmic-swingset/src/chain-main.js, and uses that for an ava snapshot test of localhost:${OTEL_EXPORTER_PROMETHEUS_PORT}/metrics. Includes incidental fixes and utility additions along the way (and therefore best reviewed by commit).

### Security Considerations
None known.

### Scaling Considerations
n/a

### Documentation Considerations
n/a

### Testing Considerations
Just the usual `--update-snapshots` when things change.

### Upgrade Considerations
n/a

packages/cosmic-swingset/package.json

@@ -34,6 +34,7 @@
     "@endo/bundle-source": "^3.5.1",
     "@endo/env-options": "^1.1.8",
     "@endo/errors": "^1.2.9",
+    "@endo/eventual-send": "^1.3.0",
     "@endo/import-bundle": "^1.3.3",
     "@endo/init": "^1.1.8",
     "@endo/far": "^1.1.10",
@@ -47,6 +48,7 @@
     "anylogger": "^0.21.0",
     "deterministic-json": "^1.0.5",
     "import-meta-resolve": "^2.2.1",
+    "ses": "^1.11.0",
     "tmp": "^0.2.1"
   },
   "devDependencies": {