Sanitize metricset tag keys

[eyalkoren]

What does this PR do?

APM Server would reject metricset tag keys if they contain disallowed characters.

Checklist

• This is an enhancement of existing features, or a new feature in existing plugins
  • I have updated CHANGELOG.asciidoc
  • I have added tests that prove my fix is effective or that my feature works
  • Added an API method or config option? Document in which version this will be introduced
  • I have made corresponding changes to the documentation
• This is a bugfix
  • I have updated CHANGELOG.asciidoc
  • I have added tests that would fail without this fix
• This is a new plugin
  • I have updated CHANGELOG.asciidoc
  • My code follows the style guidelines of this project
  • I have made corresponding changes to the documentation
  • I have added tests that prove my fix is effective or that my feature works
  • New and existing unit tests pass locally with my changes
  • I have updated supported-technologies.asciidoc
  • Added an API method or config option? Document in which version this will be introduced
  • Added an instrumentation plugin? Describe how you made sure that old, non-supported versions are not instrumented by accident.
• This is something else
  • I have updated CHANGELOG.asciidoc

[codecov-commenter]

Codecov Report

Merging #1413 into master will decrease coverage by 0.14%.
The diff coverage is 23.52%.

@@             Coverage Diff              @@
##             master    #1413      +/-   ##
============================================
- Coverage     59.83%   59.68%   -0.15%     
  Complexity       91       91              
============================================
  Files           385      385              
  Lines         17331    17333       +2     
  Branches       2374     2375       +1     
============================================
- Hits          10370    10346      -24     
- Misses         6249     6276      +27     
+ Partials        712      711       -1     

Impacted Files	Coverage Δ	Complexity Δ
.../apm/agent/report/serialize/DslJsonSerializer.java	89.20% <ø> (ø)	0.00 <0.00> (ø)
...m/agent/util/DependencyInjectingServiceLoader.java	75.64% <0.00%> (ø)	0.00 <0.00> (ø)
.../ApacheHttpAsyncClientRedirectInstrumentation.java	50.00% <0.00%> (-4.55%)	0.00 <0.00> (ø)
...t/grails/GrailsTransactionNameInstrumentation.java	36.36% <ø> (-13.64%)	0.00 <0.00> (ø)
...m/agent/scalaconcurrent/FutureInstrumentation.java	39.13% <0.00%> (ø)	0.00 <0.00> (?)
.../springwebmvc/ExceptionHandlerInstrumentation.java	71.42% <ø> (ø)	0.00 <0.00> (?)
...springwebmvc/SpringServiceNameInstrumentation.java	60.00% <ø> (ø)	0.00 <0.00> (?)
...ngwebmvc/SpringTransactionNameInstrumentation.java	36.36% <ø> (ø)	0.00 <0.00> (?)
.../agent/springwebmvc/ViewRenderInstrumentation.java	48.93% <0.00%> (ø)	0.00 <0.00> (?)
...java/co/elastic/apm/agent/bci/ElasticApmAgent.java	72.50% <33.33%> (-0.32%)	0.00 <0.00> (ø)
... and 11 more

---

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[ghost]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #1413 opened]

• Start Time: 2020-09-22T13:54:34.330+0000

• Duration: 51 min 19 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1561
Skipped	12
Total	1573

Steps errors

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• Name: Download Codecov

  • Description: #!/bin/bash set -x curl -sSLo codecov.sh https://codecov.io/bash

  • Duration: 2 min 22 sec

  • Start Time: 2020-09-22T14:43:21.585+0000

  • log