Add a mechanism to allow to specifiy the command line flags over env variables

[johscheuer]

Made some tests locally:

$ ./fdbkubernetesmonitor
{"level":"info","ts":1718374909.7578251,"msg":"should enable pprof","enable-pprof":false}

$ ENABLE_PPROF=true ./fdbkubernetesmonitor
{"level":"info","ts":1718374917.926634,"msg":"should enable pprof","enable-pprof":true}

$ ENABLE_PPROF=falsee ./fdbkubernetesmonitor
panic: strconv.ParseBool: parsing "falsee": invalid syntax

goroutine 1 [running]:
main.main()
        /Users/jscheuermann/workspace/apple/foundationdb/fdbkubernetesmonitor/main.go:130 +0xd08

$ ENABLE_PPROF=false ./fdbkubernetesmonitor
{"level":"info","ts":1718374929.785943,"msg":"should enable pprof","enable-pprof":false}

$ ENABLE_PPROF="false" ./fdbkubernetesmonitor
{"level":"info","ts":1718374937.487411,"msg":"should enable pprof","enable-pprof":false}

$ ENABLE_PPROF="false" ./fdbkubernetesmonitor --enable-pprof
{"level":"info","ts":1718374991.660543,"msg":"should enable pprof","enable-pprof":true}

Those changes allow to specify the value for a command line flag as env variable with the same name.

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[foundationdb-ci]

Result of foundationdb-pr-clang-ide on Linux CentOS 7

• Commit ID: 7a4882a2e83b14994e5c1c56cd6e0b9b202a62df
• Duration 0:21:57
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[foundationdb-ci]

Result of foundationdb-pr-macos-m1 on macOS Ventura 13.x

• Commit ID: 7a4882a2e83b14994e5c1c56cd6e0b9b202a62df
• Duration 0:37:11
• Result: :white_check_mark: SUCCEEDED
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[foundationdb-ci]

Result of foundationdb-pr-macos on macOS Ventura 13.x

• Commit ID: 7a4882a2e83b14994e5c1c56cd6e0b9b202a62df
• Duration 0:47:22
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[foundationdb-ci]

Result of foundationdb-pr-clang on Linux CentOS 7

• Commit ID: 7a4882a2e83b14994e5c1c56cd6e0b9b202a62df
• Duration 0:48:15
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[foundationdb-ci]

Result of foundationdb-pr-clang-arm on Linux CentOS 7

• Commit ID: 7a4882a2e83b14994e5c1c56cd6e0b9b202a62df
• Duration 0:51:35
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[foundationdb-ci]

Result of foundationdb-pr-cluster-tests on Linux CentOS 7

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• Duration 0:51:50
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[foundationdb-ci]

Result of foundationdb-pr on Linux CentOS 7

• Commit ID: 7a4882a2e83b14994e5c1c56cd6e0b9b202a62df
• Duration 0:55:49
• Result: :white_check_mark: SUCCEEDED
• Error: N/A
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