My own use case means I need to convert the values of tags from upper case to lo…wer case (or vice versa) because IIS logs I've been working with contain the character case originally expressed in each request, but this means statistics are not aggregated together by the basicstats aggregator.
I tried building and using the regex processor, #3839 , but regex in golang doesn't support the case transformations available in Perl with \L and \U. Everything else about the plugin was a perfect fit so I copied its structure and want to credit @44px for most of the hard work. The plugin just calls strings.ToLower on the tags and fields specified.
### Configuration Example
```
[[processors.lowercase]]
namepass = "iis_log"
[[processors.lowercase.tags]]
key = "01_uriStem"
```
### Source metric from logparser
```
iis_log,host=MYMACHINE,02_method=GET,01_uriStem=/api/Healthcheck,03_response=200 port="7314",protocolVersion="HTTP
/1.1",serverName="MYHOST",log_timestamp="2018-03-08 00:02:59",clientIP="fe80::beef:beef:feeb:9071%12",bytesSent=436i,req
uestHost="THEIRHOST:7314",referer="-",userAgent="Mozilla/5.0+(Windows+NT;+Windows+NT+6.3;+en-GB)+WindowsPowerShell/5.1.1
4409.1012",serverIP="fe80::beef:beef:beef:9071%12",timetaken=1781i,username="-",serviceName="W3SVC11",win32response="0",
uriQuery="-",bytesReceived=178i,subresponse="0" 1521122773998868100
iis_log,02_method=GET,01_uriStem=/API/healthcheck,03_response=200,host=MYMACHINE uriQuery="-",username="-",bytesSe
nt=436i,bytesReceived=178i,clientIP="fe80::beef:beef:feeb:9071%12",serverName="MYHOST",serverIP="fe80::beef:beef:beef:90
71%12",userAgent="Mozilla/5.0+(Windows+NT;+Windows+NT+6.3;+en-GB)+WindowsPowerShell/5.1.14409.1012",subresponse="0",log_
timestamp="2018-03-08 00:03:00",protocolVersion="HTTP/1.1",serviceName="W3SVC11",port="7314",timetaken=1781i,referer="-"
,requestHost="THEIRHOST:7314",win32response="0" 1521122773998868101
```
### Results
```
iis_log,host=LAPTOP-A5GCJ6G9,03_response=200,02_method=GET,01_uriStem=/api/healthcheck uriQuery="-",requestHost="THEIRHO
ST:7314",timetaken=1781i,bytesReceived=178i,userAgent="Mozilla/5.0+(Windows+NT;+Windows+NT+6.3;+en-GB)+WindowsPowerShell
/5.1.14409.1012",serverName="MYHOST",bytesSent=436i,username="-",clientIP="fe80::beef:beef:feeb:9071%12",log_timestamp="
2018-03-08 00:02:59",subresponse="0",serverIP="fe80::beef:beef:beef:9071%12",port="7314",win32response="0",referer="-",s
erviceName="W3SVC11",protocolVersion="HTTP/1.1" 1521122905846447300
iis_log,02_method=GET,03_response=200,host=LAPTOP-A5GCJ6G9,01_uriStem=/api/healthcheck subresponse="0",serviceName="W3SV
C11",protocolVersion="HTTP/1.1",serverName="MYHOST",bytesSent=436i,port="7314",serverIP="fe80::beef:beef:beef:9071%12",b
ytesReceived=178i,clientIP="fe80::beef:beef:feeb:9071%12",username="-",uriQuery="-",win32response="0",log_timestamp="201
8-03-08 00:03:00",userAgent="Mozilla/5.0+(Windows+NT;+Windows+NT+6.3;+en-GB)+WindowsPowerShell/5.1.14409.1012",requestHo
st="THEIRHOST:7314",timetaken=1781i,referer="-" 1521122905846447301
```
### Required for all PRs:
- [x] Signed [CLA](https://influxdata.com/community/cla/).
- [x] Associated README.md updated.
- [x] Has appropriate unit tests.
