nix: implicitly install pm2-prom-module on startup

[hlolli]

this is non-ideal solution to get pm2-prom-module, as it will run on every consumer of the nix package of ryo. But with flakification work down the line we'll be able to add options to run custom pre/post scripts on startup. As a temporary workaround to prevent unnecessary complexity on operator side, we'll install this package, which is in a way quite useful for any operator of ryo anyway as they'll gain more visibility and stats to their monitoring setups.

[Copilot]

Pull request overview

Updates the Nix-packaged startup wrapper for blockfrost-backend-ryo to automatically install pm2-prom-module on service start, aiming to reduce operator setup work for Prometheus/PM2 monitoring.

Changes:

• Split the PM2 startup wrapper into separate commands (delete, install module, start).
• Add pm2 install pm2-prom-module during startup (errors currently ignored).

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

pm2 delete all will remove every PM2-managed process for the service user/PM2_HOME, including any other apps or modules an operator may be running under the same user. Safer is to name this process explicitly (e.g., via pm2 start --name) and delete only that name (or use a dedicated PM2_HOME / user that is guaranteed to be exclusive).

Suggested change

	${nodePackages.pm2}/bin/pm2 delete all
	${nodePackages.pm2}/bin/pm2 install pm2-prom-module || true
	${nodePackages.pm2}/bin/pm2 start \
	$out/libexec/source/dist/server.js \
	pm2AppName="${name}"
	${nodePackages.pm2}/bin/pm2 delete "$pm2AppName" || true
	${nodePackages.pm2}/bin/pm2 install pm2-prom-module || true
	${nodePackages.pm2}/bin/pm2 start \
	$out/libexec/source/dist/server.js \
	--name "$pm2AppName" \

[Copilot]

Installing pm2-prom-module at runtime (pm2 install ...) makes startup depend on external network availability and an unpinned npm artifact, which hurts reproducibility and can break cold starts/offline deployments. If this is required, prefer packaging/pinning it in Nix (or at least pinning the version in the install command) and handle failures explicitly rather than swallowing them with || true (e.g., log and continue, or gate install on module not already being present).

[slowbackspace]

superseded by #320