Make vault clients export metrics with default values if no data is available

[ebma]

Context

When a vault client is running for a liquidated vault, some data is not exported anymore. This makes it difficult for the monitoring services to work properly as 'no data' would generally be considered a bug or faulty. To facilitate the monitoring and alerting of our vault clients we should change this to rather return a default value of instead.

TODO

We want to change the vault clients to keep exporting data for the following metrics, even if the vault is liquidated:

• Locked collateral
• Required collateral
• Issue count
• Stellar Balance
• Redeem count
• Collateralization

For the metrics that are not applicable for liquidated vaults, namely locked and required collateral as well as collateralization, we should return 0 instead. For the others, we can keep exporting the correct values.

[ebma]

@pendulum-chain/product this ticket helps improve our automated alerts as it allows to monitor vaults even when they are liquidated.

[prayagd]

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[gianfra-t]

@ebma all the listed fields show no data right now? For instance, even the redeem count for a liquidated vault.