rm-hull / fuel-prices-api / 22685973199

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feat: add prometheus metrics for api client (#21)

* feat: add prometheus metrics for api client

Add monitoring for the GOV.UK fuel finder API client using Prometheus.
This allows tracking request latency and status codes to ensure
reliability and performance visibility.

-   Introduces `ClientFetchMetrics` to manage Prometheus collectors.
-   Tracks `fuel_prices_govuk_api_http_response_latency_seconds`.
-   Tracks `fuel_prices_govuk_api_http_response_status_codes_total`.
-   Instruments `GET` and `POST` methods in `fuelPricesManager`.

```mermaid
sequenceDiagram
    participant C as FuelPricesClient
    participant M as ClientFetchMetrics
    participant A as GOV.UK API
    C->>C: Start timer
    C->>A: Execute HTTP request
    A-->>C: Return response/error
    C->>M: Record(duration, method, path, status)
```

* feat: track total items fetched via metrics

Introduces a new Prometheus counter to monitor the volume of records
retrieved from the GOV.UK fuel finder API. This provides better
observability into data ingestion performance and volume.

The change also refactors existing HTTP instrumentation to pass the
full response object, improving encapsulation of metric recording.

*   Added `fuel_prices_govuk_api_items_fetched_total` counter.
*   Renamed `Record` to `RecordHttpCall` in `ClientFetchMetrics`.
*   Integrated item count recording into the batched fetch loop.

```mermaid
sequenceDiagram
    participant C as Client
    participant M as Metrics
    participant A as API

    C->>A: HTTP Request
    A-->>C: Response
    C->>M: RecordHttpCall(method, path, resp)

    loop Batched Fetch
        C->>A: Get Batch
        A-->>C: Records
        C->>M: RecordFetchedItems(path, count)
    end
```

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