Compute and write metrics for rows prior to store writes

[zhilingc]

What this PR does / why we need it:

In PR #568, ingestion metrics were shifted to be computed and written post-store-writes, in order to accurately reflect the values that are actually available to users.

However, for certain stores (like BQ), successfully written rows cannot be retrieved post-write, and as a result feature row metrics are not produced for ingestion jobs writing to such stores. This PR re-introduces inflight feature row metrics as an alternative source of monitoring.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

Adds the tag "metrics_namespace" to the following metrics:
- feature_row_lag_ms_*
- feature_value_lag_ms_*
- feature_row_ingested_count
- feature_value_missing_count
- feature_value_*

The tag can contain either of:
- `Inflight` 
- `WriteToStoreSuccess`

[zhilingc]

/test test-end-to-end-batch

[Yanson]

Does this fix #747?

[zhilingc]

Does this fix #747?

Sort of. It provides an alternative means to monitor elements being ingested when the sink is BigQuery.

[woop]

@zhilingc can we add this change in metrics to our documentation please?

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: woop, zhilingc

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[woop]

/lgtm