Add plugin_measurement_filter

This patch initially adds a "measurement_filter" plugin, which can be
used to extract values from measurement messages and apply a filter
function on them.

If the filter matches, the plugin forwards the message to a certain
other plugin, if not it can optionally send the message to an
alternative plugin.

Signed-off-by: Matthias Beyer <matthias.beyer@ifm.com>

1 files changed, 55 insertions, 0 deletions

diff --git a/plugins/plugin_measurement_filter/README.md b/plugins/plugin_measurement_filter/README.md
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+# plugin_measurement_filter
+
+The "measurement_filter" plugin can be used to extract values from measurement
+messages and apply a filter function on them. If the filter matches, the plugin
+forwards the message to a certain plugin, if not it can optionally send the
+message to an alternative plugin.
+
+
+## Configuration
+
+The plugin configuration is made out of four values:
+
+* The target plugin to send messages to that are filtered
+* An optional alternative plugin that messages get send to that are "filtered
+  out"
+* An extractor, that must be used to extract a value from a measurement message
+  for filtering
+* A filter predicate
+
+An example would look like this:
+
+```toml
+target = "logger"
+filtered_target = "some_other_plugin" # optional
+
+# Extract the value from messages named "temperature" at field "fahrenheit"
+extractor = "temperature.fahrenheit"
+
+# Messages with fahrenheit > 70 are send to "target", all others to
+# "filtered_target" or dropped
+more_than = 70.0
+```
+
+
+## Available filter predicates
+
+```toml
+# Boolean
+is = true
+
+# Float
+less_than = 10.0
+more_than = 10.0
+
+# String
+contains = "foo"
+excludes = "foo"
+```
+
+One of them must be used, using multiple is not supported.
+
+If a filter does not match the expected type (e.g. the value to filter is a
+boolean, but you try to filter with `more_than = 10.0`) the filter
+implementation will return false.
+