RFR: 8310675: Fix -Wconversion warnings in ZGC code

[Joel Sikström]

Fixed `-Wconversion` warnings in ZGC code, either by adding an explicit type cast, changin the type of the variable or calling an equivalent method with other types. The largest change is the addition of `ZStatDurationSample`, which typecasts `Tickspan::value()` to a `uint64_t` and calls `ZStatSample` to make the code more readable.

I isolated the `-Wconversion` warnings for ZGC by adding the flag to clangd and displaying the errors in my IDE and going through each file directly associated with ZGC one by one.

Tested with tiers 1-3.

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Commit messages:
 - 8310675: Fixed -Wconversion warnings in ZGC

Changes: https://git.openjdk.org/jdk/pull/20406/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20406&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310675
  Stats: 120 lines in 33 files changed: 5 ins; 0 del; 115 mod
  Patch: https://git.openjdk.org/jdk/pull/20406.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20406/head:pull/20406

PR: https://git.openjdk.org/jdk/pull/20406