Implement log function natively

[zlin179]

Describe your changes
Add the log function similar to Graphite's log

Testing performed

• Unit Test (pass)
• Integration Test (pass)

I tested using mtcmptest with the following json

{
    "log()": "log(seriesByTag('name=jitter.g'))",
    "log(0.8)": "log(seriesByTag('name=jitter.g'),0.8)",
    "log(2.73)": "log(seriesByTag('name=jitter.g'),2.73)",
    "log(10)": "log(seriesByTag('name=jitter.g'),10)",
    "log(233333333)": "log(seriesByTag('name=jitter.g'),233333333)"
}

Basically they are the same except that the tag for log is {string: string} in mt & {string: float} in graphite, and very occasionally there's 1 bit precision difference.
link to the comparison result

Additional context
Benchmark

goos: darwin
goarch: amd64
pkg: github.com/grafana/metrictank/expr
cpu: Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
BenchmarkLog10k_1NoNulls
BenchmarkLog10k_1NoNulls-12                        39400             28115 ns/op
BenchmarkLog10k_10NoNulls
BenchmarkLog10k_10NoNulls-12                        4143            300366 ns/op
BenchmarkLog10k_100NoNulls
BenchmarkLog10k_100NoNulls-12                        342           3290549 ns/op
BenchmarkLog10k_1000NoNulls
BenchmarkLog10k_1000NoNulls-12                        37          30940075 ns/op
BenchmarkLog10k_1SomeSeriesHalfNulls
BenchmarkLog10k_1SomeSeriesHalfNulls-12            43148             27560 ns/op
BenchmarkLog10k_10SomeSeriesHalfNulls
BenchmarkLog10k_10SomeSeriesHalfNulls-12            4471            293390 ns/op
BenchmarkLog10k_100SomeSeriesHalfNulls
BenchmarkLog10k_100SomeSeriesHalfNulls-12            384           3079671 ns/op
BenchmarkLog10k_1000SomeSeriesHalfNulls
BenchmarkLog10k_1000SomeSeriesHalfNulls-12            40          29678756 ns/op
BenchmarkLog10k_1AllSeriesHalfNulls
BenchmarkLog10k_1AllSeriesHalfNulls-12             42213             28313 ns/op
BenchmarkLog10k_10AllSeriesHalfNulls
BenchmarkLog10k_10AllSeriesHalfNulls-12             4590            256792 ns/op
BenchmarkLog10k_100AllSeriesHalfNulls
BenchmarkLog10k_100AllSeriesHalfNulls-12             392           3157942 ns/op
BenchmarkLog10k_1000AllSeriesHalfNulls
BenchmarkLog10k_1000AllSeriesHalfNulls-12             37          30260522 ns/op
PASS
ok      github.com/grafana/metrictank/expr      18.915s

[zlin179]

We should change the signature to func benchmarkLog(b *testing.B, numSeries int, fn0, fn1 test.DataFunc) if #2017 gets merged first

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[zlin179]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.You have signed the CLA already but the status is still pending? Let us recheck it.

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[robert-milan]

Hi @zlin179 . Can you try signing the CLA again, or possibly click on recheck it? If that doesn't help, possibly a rebase off of master?

[zlin179]

Hi @zlin179 . Can you try signing the CLA again, or possibly click on recheck it? If that doesn't help, possibly a rebase off of master?

Sure, signed it.