Check for empty tag value before metric allocation

[lpic10]

Replaces PR #2396, also fixes #2390

• CHANGELOG.md updated (we recommend not updating this until the PR has been approved by a maintainer)
• Sign CLA (if not already signed)
• README.md updated (if adding a new plugin)

[sparrc]

@lpic10 few things I would prefer in this PR:

1. write a unit test for this case in metric_test.go
2. you don't need to check the length after escaping the tag key/values, just check the raw k,v in the for-loop.
3. I checked the code and AFAICT we don't currently mutate any of the arguments passed into the metric.New function. With that in mind, I'd prefer that we didn't mutate the tags map, which just means that we'll have to check the length of k & v twice, something like this:

	// pre-allocate exact size of the tags slice
	taglen := 0
	for k, v := range tags {
		if len(k) == 0 || len(v) == 0 {
			continue
		}
		taglen += 2 + len(escape(k, "tagkey")) + len(escape(v, "tagval"))
	}
	m.tags = make([]byte, taglen)

	i := 0
	for k, v := range tags {
		if len(k) == 0 || len(v) == 0 {
			continue
		}
		m.tags[i] = ','
		i++
		i += copy(m.tags[i:], escape(k, "tagkey"))
		m.tags[i] = '='
		i++
		i += copy(m.tags[i:], escape(v, "tagval"))
	}

[lpic10]

@sparrc applied your suggestions, looks good as it is