add error metrics

[soupi]

What

We'd like to monitor the kinds of errors we run into to measure the connector's health, so we add counters for errors according to a few categorizations.

How

We add counters for the following metrics:

/// A collection of metrics indicating errors.
#[derive(Debug, Clone)]
pub struct ErrorMetrics {
    /// the connector received an invalid request.
    invalid_request_total: IntCounter,
    /// the connector received a request using capabilities it does not support.
    unsupported_capability_total: IntCounter,
    /// the connector could not fulfill a request because it does not support
    /// certain features (which are not described as capabilities).
    unsupported_feature_total: IntCounter,
    /// the connector had an internal error.
    connector_error_total: IntCounter,
    /// the database emmited an error.
    database_error_total: IntCounter,
}

And use ndc-postgres errors to find and count said errors.

[SamirTalwar]

Looks like we don't actually use this; why is it necessary?

[soupi]

Future proofing and documentation. If you don't like it I can remove it :)

[danieljharvey]

I assume we're changing to use the new reverted-metrics commit of ndc-hub?

[soupi]

I didn't bother updating the hash because that commit is just a revert so effectively it's the same code, so I reverted the update commit.

[danieljharvey]

We have another ticket for counting failed connection acquisition, so this grouping will be helpful for that.