refactor: simplify/reformat existing grafana alerts

[amdove]

Description

• Standardized alert format across all 15 Grafana alerts with consistent WHERE/DETAILS sections
• Added tenant_name field to WorkloadConfig and Alloy external_labels for tenant identification in alerts
• Alerts now use severity emojis (🔴 CRITICAL, 🟡 WARNING) and include runbook/dashboard links

Slack alerts should now look like this:

Test plan

• Just test-python-pulumi passes (111 tests)
• Deploy to staging and verify alert format in Grafana

No new tests added since tenant_name is a cosmetic field with simple fallback logic (tenant_name or compound_name), testing would only verify basic Python language features.

Category of change

• Bug fix (non-breaking change which fixes an issue)
• Version upgrade (upgrading the version of a service or product)
• New feature (non-breaking change which adds functionality)
• Build: a code change that affects the build system or external dependencies
• Performance: a code change that improves performance
• Refactor: a code change that neither fixes a bug nor adds a feature
• Documentation: documentation changes
• Breaking change (fix or feature that would cause existing functionality to not work as expected)