Update link to metric docs in README

Signed-off-by: cwasicki <[email protected]>

Author: cwasicki

README.md

@@ -69,7 +69,7 @@ client = ReportingApiClient(server_url=SERVER_URL, auth_key=AUTH_KEY, sign_secre
 
 This method supports:
 - Selecting specific `microgrid_id` and `component_id`.
-- Choosing one or more `metrics` to retrieve. Available metrics are listed [here](https://github.com/frequenz-floss/frequenz-api-common/blob/v0.8.0/proto/frequenz/api/common/v1alpha8/metrics/metrics.proto).
+- Choosing one or more `metrics` to retrieve. Available metrics are listed [here](https://frequenz-floss.github.io/frequenz-api-common/v0.8/protobuf-reference/frequenz/api/common/v1alpha8/metrics/metrics/#frequenz-api-common-v1alpha8-metrics-Metric).
 - Defining a time range with `start_time` and `end_time`.
 - Optional downsampling using `resampling_period` (e.g., `timedelta(minutes=15)`).