feat: gateway tracking metrics ipld codec and multihash function

[vasco-santos]

As part of #18 this PR augments metrics with ipld codec and multihash function type counter. Prometheus metrics look as follows:

# HELP nftgateway_responses_by_ipld_codec_total total of responses by ipld codec.
# TYPE nftgateway_responses_by_ipld_codec_total counter
nftgateway_responses_by_ipld_codec_total{env="test",codec="raw"} 1
# HELP nftgateway_responses_by_multihash_function_total total of responses by multihash function.
# TYPE nftgateway_responses_by_multihash_function_total counter
nftgateway_responses_by_multihash_function_total{env="test",function="sha2-256"} 1

Moved from nftstorage/nft.storage#1442

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[alanshaw]

I think this would be more useful if we recorded the IPLD codec of the resolved CID of the content that gets served (the etag in the response). It is not always the CID in the subdomain/path i.e. when the request is for CID and path.

[vasco-santos]

Closing this as this should be done in Loki queries