Allow to set a custom Transport for MetricsClient

[dvaldivia]

Allows the MetricsClient to set a custom http.Transport so that custom certificates can be supported.

Additionally makes all the methods for the metrics client to receive pointers as the Struct MetricsClient had methods on both value and pointer receivers. Such usage is not recommended by the Go Documentation.

[cesnietor]

LGTM

[harshavardhana]

Wait this PR is wrong @dvaldivia we do not want to encourage SetCustomTransport

[harshavardhana]

convert the metrics_client.go NewMetricsClient(endpoint, opts) and take the Transport as part of opts.Transport = transport

[dvaldivia]

I did it this way since the madmin.Client had something similar we could update all clients at the same time to that new pattern, however madmin.Client opts doens't support transport yet @harshavardhana

[harshavardhana]

I did it this way since the madmin.Client had something similar we could update all clients at the same time to that new pattern, however madmin.Client opts doens't support transport yet @harshavardhana

Yeah new clients can use newer style @dvaldivia we have to move madmin.New()