ES output: allow to use tag values on index name

[lpic10]

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

Should implement #3442

This should have some effect on performance but I haven't measured. I'm not go expert, there may be better ways of doing the same.

Let me know if this is ok, so I update the doc as well.

[danielnelson]

If a user has an environment variable set with the same name as the tag you might have problems, since we use the same ${var} syntax for these. We could use the Go template package https://golang.org/pkg/text/template/, it would look like foo-{{.host}}, they are a little bit overkill though in terms of what you could do with them.

[lpic10]

If a user has an environment variable set with the same name as the tag you might have problems, since we use the same ${var} syntax for these.

Ops, yeah, completely forgot about that... I will have a look at the template package.

[lpic10]

Well, at the end it was much simpler to just change the notation to foo-{{host}} than using template.

[danielnelson]

If we trim the space here, then I think the tag may not match further down in the replacer.

[lpic10]

Yes, that's why I kept the the tagName variable to be used by the replacer instead of trimming on it directly

[danielnelson]

Since this happens for each metric, I'm concerned about the performance. Perhaps this can be improved with by doing some of the work at Connect time.

I'll try to explain in more detail, this part needs only done once:

• Make an array of tagkeys: ["host"]
• Build a format string from the indexName template with the tags replaced with %s: telegraf-%s-%Y.%m.%d

Then at Write time:

• Lookup the tag values and place them into an array.
• Use fmt.Sprintf with the format string and the array of tag values
• Use the existing replacer for the dates.

[lpic10]

@danielnelson I tried to implement the changes you requested, let me know.

[danielnelson]

Code looks perfect, can you rebase on master to fix the file conflict?