app/vlinsert: consistently increase vl_http_errors_total on failed inserts

[func25]

Previously vl_http_errors_total doesn't increase when inserts were rejected because vlstorage was in read-only mode (e.g. out of disk space), so users had to compute failures as vl_http_requests_total - vl_http_request_duration_seconds_count.

• increase vl_http_errors_total when CanWriteData() fails for jsonline, native, multitenant native, internal insert, splunk and opentelemetry (interestingly journald already does this)
• add the missing vl_http_errors_total for loki, elasticsearch and datadog

I keeps the minimal fix, we can consider unifying error handling, but the problem is jsonline counts error per invalid log line inside a request, not per request. Not sure this is expected.

[cubic-dev-ai]

All reported issues were addressed across 11 files

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[cubic-dev-ai]

All reported issues were addressed across 3 files (changes from recent commits).

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