VED-992: Metrics for mesh backlog

[Akol125]

Summary

This PR improves observability of the MESH ingestion pipeline by introducing explicit CloudWatch metrics and alarms to detect when files are not being processed as expected.

It introduces custom, event-based metrics to record inbound and outbound file processing events. These metrics are aggregated in CloudWatch using standard statistics (e.g. Sum) to derive total inbound and outbound counts over a given time window. Alarms then compare these aggregated values to identify scenarios where incoming files exceed processed files, indicating a potential backlog or underlying processing issue within the MESH pipeline.

Reviews Required

• Dev
• Test
• Tech Author
• Product Owner

Review Checklist

ℹ️ This section is to be filled in by the reviewer.

• I have reviewed the changes in this PR and they fill all of the acceptance criteria of the ticket.
• If there were infrastructure, operational, or build changes, I have made sure there is sufficient evidence that the changes will work.
• If there were changes that are outside of the regular release processes e.g. account infrastructure to setup, manual setup for external API integrations, secrets to set, then I have checked that the developer has flagged this to the Tech Lead as release steps.
• I have checked that no Personal Identifiable Data (PID) is logged as part of the changes.

[github-actions]

This branch is working on a ticket in the NHS England VED JIRA Project. Here's a handy link to the ticket:

VED-992

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud