Hot reload to override individual metrics

[anders-swanson]

Override Existing, Individual Metrics

You may override properties for existing metrics by supplying a new, custom metric definition with the same context and metricsdesc values. For example, if you have an existing metric like so:

[[metric]]
context = "my_default_metric"
metricsdesc = { value_1 = "Simple example returning always 1.", value_2 = "Same but returning always 2." }
request = "SELECT 1 as value_1, 2 as value_2 FROM DUAL"

You can redefine this metric in a custom metrics file to change any properties other than context or metricsdesc. For example, overriding the previous metric with labels, scrapeinterval, and querytimeout properties:

[[metric]]
context = "my_default_metric"
metricsdesc = { value_1 = "Simple example returning always 1.", value_2 = "Same but returning always 2." }
labels = [ "label_1", "label_2" ]
request = "SELECT 1 as value_1, 2 as value_2 FROM DUAL"
scrapeinterval = "30s"
querytimeout = "10s"

Then, provide any metrics overrides as custom metrics files in the exporter configuration file:

metrics:
  ## Paths to any custom metrics files
  custom:
    - my-custom-metrics.toml

If any metric appears more than once in the custom metrics file list, the metric definition in the last file provided takes precedence.

[anders-swanson]

Previously, behavior for duplicate metrics was undefined. This also fixes that issue, merge-replacing duplicate metrics.

[anders-swanson]

Fixes #375