Fix metricBucket losing object size in Upload

[andyasp]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

In metricBucket's Upload the reader was being wrapped with io.NopCloser, but newTimingReadCloser will call TryToGetSize here and then get an error.

I noticed this because I saw this in a test's log message (reference)

21:56:01 alertmanager-1: ts=2023-09-18T21:56:01.855369927Z caller=s3.go:487 level=warn msg="could not guess file size for multipart upload; upload might be not optimized" name=alertmanager/user-1/fullstate err="unsupported type of io.Reader: io.nopCloserWriterTo"

Introduced in #66 (comment)

Verification

Stepped through a throwaway test:

b := WrapWithMetrics(NewInMemBucket(), prometheus.NewPedanticRegistry(), "a")
b.Upload(context.Background(), "test", bytes.NewReader([]byte{}))