feat: Add "longFieldMaxLength" config var

[trentm]

elastic/apm#493 specifies a long_field_max_length config option we can use for span.context.db.statement. Currently this is hardcoded to 10000... bytes.

• We'll also change to be 10000 unicode characters (rather than bytes). This is in line with the json-schema spec for "maxLength" and is now the apm spec for long_field_max_length. See the http-client PR for this part of the change.
• This also deprecates the existing node.js-agent-only errorMessageMaxLength config option. It now defaults to the longFieldMaxLength value if not specified. Otherwise errorMessageMaxLength is still supported. (We should drop it in the "next major".)
• This change also applies the truncation to the other fields mentioned in the long_field_max_length spec. This does mean that some existing usage out there might get truncation of some of these fields where they didn't before. That's debatable whether that constitutes a "breaking change". So far this PR takes the point of view that this is not a breaking change that would require either a separate config var (which means not conforming with the APM spec link above) or only applying truncation to these fields in a new major version later. Objections?

Fixes: #1921

Checklist

• Implement code
• update package.json dep once feat: truncation changes apm-nodejs-http-client#162 is merged and v10.0.0 is published
• Add tests
• Update TypeScript typings
• Update documentation
• Add CHANGELOG.asciidoc entry
• Commit message follows commit guidelines

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• Start Time: 2021-09-02T21:49:33.761+0000

• Duration: 19 min 33 sec

• Commit: 055769b

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[trentm]

elastic/apm#488 is a discussion for a cross-agent config option name for this var. Let's hold until that discussion plays out.

[trentm]

I've started elastic/apm#493 for the spec addition for this field name: long_field_max_length.

[trentm]

^^ Integration Tests failure is the current elastic/apm-integration-testing#1188 flake

[trentm]

Here is a small test script to demonstrate longFieldMaxLength working on span.context.db.statement:

const apm = require('./').start({
  longFieldMaxLength: 100000
})

const { Client } = require('@elastic/elasticsearch')
var client = new Client({
  node: 'http://localhost:9200',
  auth: { username: 'admin', password: 'changeme' }
})

const t1 = apm.startTransaction('t1')
// Build up a search body >10k in size.
aggs = {}
for (var i = 0; i < 1000; i++) {
  aggs[`aggs${i}`] = { "terms": { "field": "author" } }
}
client.search(
    {
      body: {
        size: 1,
        aggregations: aggs
      }
    },
    function (err, result) {
      t1.end()
    }
)

[astorm]

Overall looks it does the thing the way the agent wants the thing done. Will approve once the package.json is updated to reflect what we want to ship.