Add strings processor (#4476)

plugins/processors/strings/README.md

@@ -0,0 +1,83 @@
+# Strings Processor Plugin
+
+The `strings` plugin maps certain go string functions onto measurement, tag, and field values.  Values can be modified in place or stored in another key.
+
+Implemented functions are:
+- lowercase
+- uppercase
+- trim
+- trim_left
+- trim_right
+- trim_prefix
+- trim_suffix
+
+Please note that in this implementation these are processed in the order that they appear above.
+
+Specify the `measurement`, `tag` or `field` that you want processed in each section and optionally a `dest` if you want the result stored in a new tag or field. You can specify lots of transformations on data with a single strings processor.
+
+### Configuration:
+
+```toml
+[[processors.strings]]
+  # [[processors.strings.uppercase]]
+  #   tag = "method"
+
+  # [[processors.strings.lowercase]]
+  #   field = "uri_stem"
+  #   dest = "uri_stem_normalised"
+
+  ## Convert a tag value to lowercase
+  # [[processors.strings.trim]]
+  #   field = "message"
+
+  # [[processors.strings.trim_left]]
+  #   field = "message"
+  #   cutset = "\t"
+
+  # [[processors.strings.trim_right]]
+  #   field = "message"
+  #   cutset = "\r\n"
+
+  # [[processors.strings.trim_prefix]]
+  #   field = "my_value"
+  #   prefix = "my_"
+
+  # [[processors.strings.trim_suffix]]
+  #   field = "read_count"
+  #   suffix = "_count"
+```
+
+#### Trim, TrimLeft, TrimRight
+
+The `trim`, `trim_left`, and `trim_right` functions take an optional parameter: `cutset`.  This value is a string containing the characters to remove from the value.
+
+#### TrimPrefix, TrimSuffix
+
+The `trim_prefix` and `trim_suffix` functions remote the given `prefix` or `suffix`
+respectively from the string.
+
+### Example
+**Config**
+```toml
+[[processors.strings]]
+  [[processors.strings.lowercase]]
+    field = "uri-stem"
+
+  [[processors.strings.trim_prefix]]
+    field = "uri_stem"
+    prefix = "/api/"
+
+  [[processors.strings.uppercase]]
+    field = "cs-host"
+    dest = "cs-host_normalised"
+```
+
+**Input**
+```
+iis_log,method=get,uri_stem=/API/HealthCheck cs-host="MIXEDCASE_host",referrer="-",ident="-",http_version=1.1,agent="UserAgent",resp_bytes=270i 1519652321000000000
+```
+
+**Output**
+```
+iis_log,method=get,uri_stem=healthcheck cs-host="MIXEDCASE_host",cs-host_normalised="MIXEDCASE_HOST",referrer="-",ident="-",http_version=1.1,agent="UserAgent",resp_bytes=270i 1519652321000000000
+```