fix(bigtable): Error logging for client side metrics

[bhshkh]

1. Use Bigtable client options while creating Metrics client. For e.g. if service account key is getting used to create Bigtable client, same will be used for GCM client. E.g.

        serviceAccountKeyFile := "key.json"
	credOpt := option.WithCredentialsFile(serviceAccountKeyFile)
	adminClient, err = bigtable.NewAdminClient(ctx, project, instance, credOpt)

2. Prefixed the errors with "bigtable-metrics: " Unless the user has set their own error handler, error log will be of the format below:

2024/08/28 15:06:46 bigtable-metrics: rpc error: code = PermissionDenied desc = Permission monitoring.timeSeries.create denied (or the resource may not exist).

3. Moving tests for metrics_util.go into their own file.

[bhshkh]

@igorbernstein2 As discussed, I have removed all the log writes and just prefixed the error messages with bigtable-metrics