[Parse incomplete chunks 8/9] Add prometheus metrics for bytes rendered unparseable before gap

[benkilimnik]

Summary: Adds metric to keep track of the number of bytes rendered unparseable due to the presence of a gap (i.e. the bytes we discard because we can't process the incomplete chunk). This is used in a future PR to lazily parse up until the gap only if we know there is a gap coming (to avoid masking unrelated errors).

Type of change: /kind feature

Test Plan: Tested that metrics work in a bpf test introduced in # and that they show up in BigQuery.