feat(gw): add ipfs_http_gw_request_types metric

[lidel]

This PR adds an explicit request type metric, a counter of how many requests of a specific format hit the gateway.
When no explicit format is requested, the type is empty string "".

This is useful on its own, allowing gateway operators to reason what request types are the most popular, but also helps with debugging described in #288.

[codecov]

Codecov Report

Merging #311 (498d5a6) into main (4fb455d) will decrease coverage by 9.26%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##             main     #311      +/-   ##
==========================================
- Coverage   57.42%   48.16%   -9.26%     
==========================================
  Files         232      279      +47     
  Lines       27739    33511    +5772     
==========================================
+ Hits        15928    16140     +212     
- Misses      10109    15684    +5575     
+ Partials     1702     1687      -15     

Impacted Files	Coverage Δ
gateway/metrics.go	60.69% <86.95%> (+2.93%)	⬆️
gateway/handler.go	57.56% <100.00%> (+0.07%)	⬆️

... and 61 files with indirect coverage changes

[Jorropo]

This is a non trivial change, I am surprised this was merged without review or comment explaining why merging as-is.

[lidel]

This is a trivial change -- it only adds new counter metric.

[Jorropo]

LGTM

[Jorropo]

It's code that do stuff. I don't think that trivial. You could had made a wrong copy paste and did i.getMetric.WithLabelValues(contentPath.Namespace(), responseFormat).Inc() instead (I know my example wouldn't work because getMetric is a histogram, it's more to show who knows what could happen in 20 lines of code ?).

Having someone quickly read with a different state of mind can catch simple mistakes that happen when you code quickly.
Anyway this looks good.