modify args of logger so that it follows key/value pairs（Fixes #303 partially）

[sshota0809]

This PR includes "Fixes #303 (Partially)".

As I said in #303, when an error occurs in pkg/plugin.go, err object isn't expanded to %v. So I can't see detail error.

Example:

bigquery-grafana/pkg/plugin.go

Line 187 in 756780e

log.DefaultLogger.Info("Query run error: %v\n", err)

t=2021-01-06T11:53:01+0000 lvl=info msg="Query run error: %v\n" logger=plugins.backend pluginId=doitintl-bigquery-datasource

Logger in github.com/grafana/grafana-plugin-sdk-go/backend/log uses go-hclog(https://github.com/hashicorp/go-hclog). From README in go-hclog, args of logger should be specified as key/value pair.
https://github.com/hashicorp/go-hclog#emit-an-info-level-message-with-2-keyvalue-pairs

So I have modified args and confirmed err is expanded.

t=2021-01-08T00:34:22+0000 lvl=info msg="Query run error" logger=plugins.backend pluginId=doitintl-bigquery-datasource Error="googleapi: Error 404: Not found: Project a, notFound"

Would you check codes that I have modified? Thank you for looking my PR!