Add FormatStatsPath to ochttp Transport to reduce stats path cardinality

[otternq]

Updates ochttp.Transport to accept an optional FormatStatsPath which is supplied to ochttp.statsTransport. FormatStatsPath can override a request URL's path parameter to reduce cardinality for stats similar to FormatSpanName.

This should be useful when a resource id is part of a request path such as /posts/1 or /posts/2. Currently each resource is treated as a unique path but it would be nice to optionally treat these requests as a single path.

Without FormatStatsPath specified

2020/10/01 09:02:59     Row: 0: &view.Row{
    Tags:[]tag.Tag{
        tag.Tag{Key:tag.Key{name:"http_client_host"}, Value:"jsonplaceholder.typicode.com"}, 
        tag.Tag{Key:tag.Key{name:"http_client_method"}, Value:"GET"}, 
        tag.Tag{Key:tag.Key{name:"http_client_path"}, Value:"/posts/2"}, 
        tag.Tag{Key:tag.Key{name:"http_client_status"}, Value:"200"}
    }, 
    Data:(*view.CountData)(0xc00035c080)
}
2020/10/01 09:02:59     Row: 1: &view.Row{
    Tags:[]tag.Tag{
        tag.Tag{Key:tag.Key{name:"http_client_host"}, Value:"jsonplaceholder.typicode.com"}, 
        tag.Tag{Key:tag.Key{name:"http_client_method"}, Value:"GET"}, 
        tag.Tag{Key:tag.Key{name:"http_client_path"}, Value:"/posts/1"}, 
        tag.Tag{Key:tag.Key{name:"http_client_status"}, Value:"200"}
    }, 
    Data:(*view.CountData)(0xc00035c100)
}

With FormatStatsPath specified

2020/10/01 09:06:48     Row: 0: &view.Row{
    Tags:[]tag.Tag{
        tag.Tag{Key:tag.Key{name:"http_client_host"}, Value:"jsonplaceholder.typicode.com"}, 
        tag.Tag{Key:tag.Key{name:"http_client_method"}, Value:"GET"}, 
        tag.Tag{Key:tag.Key{name:"http_client_path"}, Value:"/posts/:id"}, 
        tag.Tag{Key:tag.Key{name:"http_client_status"}, Value:"200"}
    }, 
    Data:(*view.DistributionData)(0xc0001181b0)
}

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[otternq]

@googlebot I signed it!

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