chore: add metrics documentation

metrics/README.md

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+# Pricefeeder Metrics
+
+## Available metrics
+
+### `fetched_prices_total`
+
+The total number of prices fetched from the data sources. This metric is incremented every time the price feeder fetches prices from the data sources.
+
+**labels**:
+
+- `source`: The data source from which the price was fetched, e.g. `Bybit`.
+- `success`: The result of the fetch operation. Possible values are 'true' and 'false'.
+
+### `aggregate_prices_total`
+
+The total number of times the `AggregatePriceProvider` is called to return a price. It randomly selects a source for each pair from its map of price providers. This metric is incremented every time the `AggregatePriceProvider` is called.
+
+**labels**:
+
+- `pair`: The pair for which the price was aggregated.
+- `source`: The data source from which the price was fetched, e.g. `Bybit`.
+- `success`: The result of the fetch operation. Possible values are 'true' and 'false'.
+
+### `prices_posted_total`
+
+The total number of txs sent to the on-chain oracle module. This metric is incremented every time the price feeder posts a price to the on-chain oracle module.
+
+**labels**:
+
+- `success`: The result of the post operation. Possible values are 'true' and 'false'.