fix[cortextool]: fix cortextool out of bounds when running export

[LERUfic]

Hello everyone,

Just for background, cortextool has a similar issue to the one reported in mimirtool here: grafana/mimir#6466. it occurs when performing exports.

# ./cortextool remote-read export --address "http://<omit>" --tsdb-path "./fuse-backup/cortex-20250408" --from="2025-04-08T06:00:00+07:00" --id="<omit>" --to="2025-04-08T19:00:00+07:00"
INFO[0000] Created remote read client using endpoint 'http://<ommit>/prometheus/api/v1/read'
INFO[0000] Using existing TSDB in path './fuse-backup/cortex-20250408'
INFO[0000] Querying time from=2025-04-08T06:00:00+07:00 to=2025-04-08T19:00:00+07:00 with selector=up
INFO[0004] Store TSDB blocks in './fuse-backup/cortex-20250408'
INFO[0019] BLOCK ULID                  MIN TIME                       MAX TIME                       DURATION     NUM SAMPLES  NUM CHUNKS   NUM SERIES   SIZE
INFO[0019] 01JRY5XGG1RTAMV2HHY29E0FH9  2025-04-07 23:00:00 +0000 UTC  2025-04-08 00:00:00 +0000 UTC  1h0m0s       789564       6643         3984         1MiB833KiB739B
cortextool: error: process blocks: add sample for metric={__name__="up",<omit>} ts=2025-04-08T07:20:21.247+07:00 value=1.000000: out of bounds, try --help

This issue has already been resolved in Mimir in the following pull request: grafana/mimir#5700. In this PR, I’m simply replicating the changes from that one. Since the idea isn't originally mine, if submitting this PR is considered inappropriate, I’m happy to close it.

Thank you

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