Add metrics for account cache

[laurb9]

Follow-up on #1519, issue #1395

Testing:
At server start:

$ curl -s http://localhost:8001/ | egrep ^account
account_cache_performance{cache_result="hit"} 0
account_cache_performance{cache_result="miss"} 0

After first request (publisher.id=valid_acct)

$ curl -s http://localhost:8001/ | egrep ^account
account_cache_performance{cache_result="hit"} 0
account_cache_performance{cache_result="miss"} 1
account_requests{account="valid_acct"} 1

After next 2 requests

$ curl -s http://localhost:8001/ | egrep ^account
account_cache_performance{cache_result="hit"} 2
account_cache_performance{cache_result="miss"} 1
account_requests{account="valid_acct"} 3

[laurb9]

Resolved conflicts with #1515 and rebased.

[SyntaxNode]

Looks great. I just want to run locally and verify metric collection. I'll do that tomorrow.

[SyntaxNode]

Tested locally and looks good.

I realize that the cache miss counter continues to increase with an invalid account id. Beyond the scope of this PR, but we might want to consider adding a cache for invalid accounts too.

[bsardo]

LGTM. I would have preferred to have seen a pure UT in go_metrics_test.go but you've got the influx metrics engine coverage through metrics_test.go.