Expose option to provide MetricServiceSettings in MetricConfiguration

[psx95]

Description

This PR adds a configuration option to the MetricConfiguration that allows the user to configure the underlying instance of MetricServiceClient which is used by the exporter to interact with the Cloud Monitoring API.

This feature is intended for advanced use-cases only.

Testing

• Added unit tests pass.
• Updated the metrics sample to demonstrate a use-case for the setMetricServiceSettings function.

fixes. #149

[dashpole]

Did you explore making these functions work with or update MetricServiceSettings?

[psx95]

Yes, I did try to reconcile these functions with the setMetricServiceSettings, but it lead to a confusing API. All of these functions eventually affect the properties of the underlying MetricServiceSettings object used to create the client.

So in a config like:

    return MetricConfiguration.builder()
        .setMetricServiceSettings(metricServiceSettingsBuilder.build())
        .setCredentials(validCredentials)
        // sets the channel to use NoCredentialsProvider and plainText.
        .setInsecureEndpoint(true)
        .setMetricServiceEndpoint(someEndpoint)
        .build();

It would become confusing for the user to predict/understand what the final configuration holds. Ignoring these other settings when setMetricServiceSettings is used keeps the implementation clean and the intent of the API clear to the user.

We also have exposed the defaults which the users can plug-in when constructing the MetricServiceSettings object.