Request counters for 429 could tell what's driving the 429

[phbnf]

429 can be driven by:

• too many duplicates in flights
• too big of an antispam lag
• too big of a sequencing queue

It would be great for TesseraCT request counters to tell the difference between them. tessera.appender.calls already makes the distinction between them, but not tesseract.http metrics.