[BUG] Race condition in metric.DefaultMetricSearcher

Issue Description

Type: bug report

Describe what happened

Race condition was detected in metric.DefaultMetricSearcher:

WARNING: DATA RACE
Read at 0x00c000510f00 by goroutine 59:
  github.com/alibaba/sentinel-golang/core/log/metric.(*DefaultMetricSearcher).getOffsetStartAndFileIdx()
      /Users/sczyh30/go/pkg/mod/github.com/alibaba/sentinel-golang@v1.0.3/core/log/metric/searcher.go:101 +0x106
  github.com/alibaba/sentinel-golang/core/log/metric.(*DefaultMetricSearcher).searchOffsetAndRead()
      /Users/sczyh30/go/pkg/mod/github.com/alibaba/sentinel-golang@v1.0.3/core/log/metric/searcher.go:71 +0x12a
  github.com/alibaba/sentinel-golang/core/log/metric.(*DefaultMetricSearcher).FindByTimeAndResource()
      /Users/sczyh30/go/pkg/mod/github.com/alibaba/sentinel-golang@v1.0.3/core/log/metric/searcher.go:53 +0xc7
   ...

Previous write at 0x00c000510f00 by goroutine 31:
  github.com/alibaba/sentinel-golang/core/log/metric.(*DefaultMetricSearcher).findOffsetToStart()
      /Users/sczyh30/go/pkg/mod/github.com/alibaba/sentinel-golang@v1.0.3/core/log/metric/searcher.go:165 +0x5ca
  github.com/alibaba/sentinel-golang/core/log/metric.(*DefaultMetricSearcher).searchOffsetAndRead()
      /Users/sczyh30/go/pkg/mod/github.com/alibaba/sentinel-golang@v1.0.3/core/log/metric/searcher.go:80 +0x1fd
  github.com/alibaba/sentinel-golang/core/log/metric.(*DefaultMetricSearcher).FindByTimeAndResource()
      /Users/sczyh30/go/pkg/mod/github.com/alibaba/sentinel-golang@v1.0.3/core/log/metric/searcher.go:53 +0xc7
  ...

Describe what you expected to happen

No race condition.

How to reproduce it (as minimally and precisely as possible)

Run a demo and invoke the search function of the MetricSearcher concurrently.

Tell us your environment

Sentinel Go v1.0.3

Anything else we need to know?

NONE