This change adds the new metric config option `expression` which can
define a complex expression to calculate the final metric value. The
expression is evaluated using github.com/antonmedv/expr as the engine,
see expr-lang.org/docs/v1.9/Language-Definition for a language overview.
In addition, some basic functions are available during evaluation: now,
int, float, round, ceil, floor, abs, min, max.
For each evaluation, the expression gets:
- the current MQTT value
- the last MQTT value
- the result of the last evaluation
- the elapsed time since the last evaluation
Expressions allow more flexibility to derive metrics from sensor inputs
which are very hard or even impossible to do in Prometheus, for example,
integrate continuous values over time, filter extreme or zero values, or
non-linear scaling. The last value, result, and timestamp are preserved
even between restarts of mqtt2prometheus. This means that expressions
can be used to produce continuous time series over a long time range.
To make upstream integration easier, the code depends on the rather old
v1.8.9 of github.com/antonmedv/expr, which is available in the main repo
of all supported Debian versions.
As mentioned in https://github.com/hikhvar/mqtt2prometheus/issues/23, we
do not use any logging framework at all. This was fine for getting the
exporter startet. However, with inreasing load the logging must be
configureable. This PR is a start to replace all instances of
"log.Printf" with the zap logger. The current configuration parameters
are the log level and the log format (console, json). We might expose
the log configuration to the config file. But I think this is overkill
for the current state of the exporter.
