add guide for migrating from 2.x to 3.0

[rochdev]

What does this PR do?

Add guide for migrating from 2.x to 3.0.

Motivation

3.0 is around the corner.

[github-actions[bot]]

Overall package size

Self size: 2.58 MB Deduped: 29.72 MB No deduping: 30.35 MB

Dependency sizes

name	version	self size	total size
@datadog/pprof	1.0.2	8.74 MB	14.46 MB
@datadog/native-metrics	1.4.2	6.61 MB	7.04 MB
@datadog/native-appsec	1.2.1	5.1 MB	5.43 MB
opentracing	0.14.7	194.81 kB	194.81 kB
@datadog/sketches-js	2.0.0	105.44 kB	105.44 kB
lodash.sortby	4.7.0	75.76 kB	75.76 kB
semver	5.7.1	61.58 kB	61.58 kB
ignore	5.2.0	48.87 kB	48.87 kB
import-in-the-middle	1.3.0	30.3 kB	34.77 kB
retry	0.10.1	27.44 kB	27.44 kB
lodash.uniq	4.5.0	25.01 kB	25.01 kB
limiter	1.1.5	23.17 kB	23.17 kB
lodash.kebabcase	4.1.1	17.75 kB	17.75 kB
lodash.pick	4.4.0	16.33 kB	16.33 kB
crypto-randomuuid	1.0.0	11.18 kB	11.18 kB
diagnostics_channel	1.1.0	7.07 kB	7.07 kB
path-to-regexp	0.1.7	6.78 kB	6.78 kB
koalas	1.0.2	6.47 kB	6.47 kB
methods	1.1.2	5.29 kB	5.29 kB
module-details-from-path	1.0.3	4.47 kB	4.47 kB

_{🤖 This report was automatically generated by heaviest-objects-in-the-universe}

[codecov[bot]]

Codecov Report

Merging #2280 (1284d58) into master (c88c058) will decrease coverage by 0.05%. The diff coverage is n/a.

:exclamation: Current head 1284d58 differs from pull request most recent head cea0dd8. Consider uploading reports for the commit cea0dd8 to get more accurate results

@@            Coverage Diff             @@
##           master    #2280      +/-   ##
==========================================
- Coverage   92.92%   92.86%   -0.06%     
==========================================
  Files         217      218       +1     
  Lines        8349     8423      +74     
==========================================
+ Hits         7758     7822      +64     
- Misses        591      601      +10     

Impacted Files	Coverage Δ
packages/dd-trace/src/plugins/util/exec.js	50.00% <0.00%> (-33.34%)	:arrow_down:
packages/dd-trace/src/plugins/plugin.js	41.93% <0.00%> (-16.13%)	:arrow_down:
packages/dd-trace/src/plugins/log_plugin.js	22.72% <0.00%> (-13.64%)	:arrow_down:
...ackages/datadog-instrumentations/src/paperplane.js	90.24% <0.00%> (-9.76%)	:arrow_down:
packages/datadog-instrumentations/src/pg.js	82.92% <0.00%> (-4.88%)	:arrow_down:
packages/dd-trace/src/plugin_manager.js	98.33% <0.00%> (-1.67%)	:arrow_down:
packages/datadog-instrumentations/src/hapi.js	97.56% <0.00%> (-1.39%)	:arrow_down:
packages/dd-trace/src/proxy.js	89.06% <0.00%> (-0.65%)	:arrow_down:
packages/dd-trace/src/telemetry.js	95.52% <0.00%> (-0.37%)	:arrow_down:
packages/dd-trace/src/plugins/index.js	100.00% <0.00%> (ø)
... and 9 more

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