fix: pre-register auto top-up Prometheus counters

[pjb157]

Summary

• Pre-registers all four dwctl_auto_topup_* counters at the top of process_auto_topups() so they appear in Prometheus as 0 from the first poll cycle
• Fixes two metrics (credit_failures_total, errors_total) that were invisible to Prometheus because their error paths have never been triggered in production

Problem

The metrics crate lazily registers counters — they only exist in the /metrics endpoint after the first .increment() call. Since dwctl_auto_topup_credit_failures_total (P1: user charged but credits not recorded) and dwctl_auto_topup_errors_total (idempotency/payment method lookup failures) have never fired in production, they don't exist in Prometheus at all.

This causes two issues:

1. Grafana alerts referencing these metrics evaluate to "no data" instead of 0, so the P1 credit-failure alert and P2 infra-failure alert would never fire even if the error paths were hit for the first time
2. Dashboard panels that divide by a sum of all auto top-up counters return NaN because part of the denominator is missing

Fix

Obtain Counter handles (without incrementing) at the top of process_auto_topups(). This registers them with the Prometheus exporter as 0 on the first poll cycle. The counters then exist continuously and will correctly increment when error paths are hit.

let _ = counter!("dwctl_auto_topup_success_total");
let _ = counter!("dwctl_auto_topup_charge_failures_total");
let _ = counter!("dwctl_auto_topup_credit_failures_total");
let _ = counter!("dwctl_auto_topup_errors_total", "stage" => "idempotency_check");
let _ = counter!("dwctl_auto_topup_errors_total", "stage" => "payment_method_lookup");

Test plan

• cargo check passes cleanly (no warnings)
• After deploy, verify all four dwctl_auto_topup_* metrics appear in Prometheus with value 0
• Verify Grafana alerts for auto top-up evaluate to "OK" (not "no data")
• Verify the Operations Overview dashboard "Auto Top-Up Activity" panel renders without NaN

[cloudflare-workers-and-pages]

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Branch Preview URL:	https://fix-pre-register-auto-topup.control-layer.pages.dev

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[Copilot]

Pull request overview

This PR ensures auto top-up Prometheus counters are visible immediately (as 0) by pre-registering the dwctl_auto_topup_* counter series when process_auto_topups() starts, preventing “no data” alert/panel behavior in Grafana/Prometheus when certain error paths haven’t occurred yet.

Changes:

• Pre-register dwctl_auto_topup_success_total, dwctl_auto_topup_charge_failures_total, dwctl_auto_topup_credit_failures_total, and dwctl_auto_topup_errors_total{stage=...} counter series at the start of process_auto_topups().
• Ensures the two previously never-triggered error counters are exported from the first poll cycle onward.