Ensure exchange rate is stringified before passing to bignumber in token confirm component

[danjm]

With this change, we introduced the possibility of exchange rate numbers having more than 15 significant digits: MetaMask/core#585

Our current version of BigNumber does not handle that well, but does if the number is stringified.

Currently in prod, users will hit an error screen when this happens. Here is the associated error record in sentry https://sentry.io/organizations/metamask/issues/2745103173/?project=273505&query=is%3Aunresolved+firstRelease%3Alatest&statsPeriod=14d

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[Gudahtt]

Do we know why bignumber.js throws on numbers with more than 15 significant digits? If it's a matter of significant digits beyond 15 being unreliable due to floating point error, maybe we're better off truncating our conversion rate to 15 (e.g. using toPrecision(15)) rather than keeping this additional precision.

Longer-term we should consider using BigInt to perform these calculations in the controllers repo so we don't lose precision from floating point errors, and truncating the calculated value to reflect the amount of precious that was input, and returning the value as a BigInt or at least a string.

[metamaskbot]

Builds ready [d5d50a1]

• builds: chrome, firefox, opera
• build viz: Build System
• code coverage: Report
• storybook: Storybook
• all artifacts
• bundle viz:

  • background: 0, 1, 2, 3, 4, 5, 6, 7, 8
  • common: 0, 1, 2, 3
  • content-script: 0
  • contentscript: contentscript
  • disable-console: disable-console
  • inpage: inpage
  • phishing-detect: phishing-detect
  • policy-load: policy-load
  • sentry-install: sentry-install
  • ui: 0, 1, 2, 3, 4, 5

Page Load Metrics (329 ± 37 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	275	542	329	78	37
		domContentLoaded	266	532	318	78	37
		load	274	540	329	78	37
		domInteractive	266	532	318	78	37

highlights:

storybook

• ui/pages/confirm-send-token/confirm-send-token.stories.js
• ui/pages/confirm-token-transaction-base/confirm-token-transaction-base.stories.js

[Gudahtt]

I'll approve for now since this seems like a quick and relatively harmless way to fix the crash, but we should follow up with a better solution that doesn't include spurious digits that give a false impression of how precise this value is.

For more information on that, see https://en.wikipedia.org/wiki/Significant_figures#Rules_to_identify_significant_figures_in_a_number
and https://en.wikipedia.org/wiki/Significant_figures#Multiplication_and_division

[danjm]

@Gudahtt A couple notes regarding big number. In the latest versions, it does not throw when significant digits are greater than 15, unless the DEBUG flag is set to true.

Before the latest version, stringifying before passing to bignumber was the recommended approach: MikeMcl/bignumber.js#148 (comment)