Add time to first byte metric for s3

[SmsS4]

What problem are we solving?

Lack of time to first byte metric for s3

How are we solving the problem?

Add a histogram metric for s3 time to first byte and observe it when filer returns response

How is the PR tested?

Simple write and read to s3 interface and observing SeaweedFS_s3_time_to_first_byte_seconds_bucket metric

Checks

• I have added unit tests if possible.
• I will add related wiki document changes and link to this PR after merging.

[chrislusf]

Millisecond is better?

[SmsS4]

Changed second to millisecond

stats_collect.S3TimeToFirstByteHistogram.WithLabelValues(action, bucket).Observe(float64(time.Since(start).Milliseconds()))

and used prometheus.ExponentialBuckets(0.001, 2, 27) for buckets.

SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="0.001"} 0
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="0.002"} 0
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="0.004"} 0
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="0.008"} 0
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="0.016"} 0
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="0.032"} 0
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="0.064"} 0
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="0.128"} 0
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="0.256"} 0
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="0.512"} 0
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="1.024"} 0
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="2.048"} 2
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="4.096"} 6
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="8.192"} 10
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="16.384"} 12
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="32.768"} 14
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="65.536"} 14
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="131.072"} 14
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="262.144"} 14
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="524.288"} 14
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="1048.576"} 14
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="2097.152"} 14
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="4194.304"} 14
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="8388.608"} 14
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="16777.216"} 14
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="33554.432"} 14
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="67108.864"} 14
SeaweedFS_s3_time_to_first_byte_millisecond_bucket{bucket="test",type="GET",le="+Inf"} 14
SeaweedFS_s3_time_to_first_byte_millisecond_sum{bucket="test",type="GET"} 110
SeaweedFS_s3_time_to_first_byte_millisecond_count{bucket="test",type="GET"} 14

[chrislusf]

please adjust the grafana dashboard also.