[oximeter] Use fmt::Display for TOML errors

[hawkw]

Currently, the oximeter::use_timeseries! macro uses fmt::Debug to display errors generating timeseries types, such as errors parsing the TOML schema. This results in kind of hard to interpret error output. For example:

error: Failed to generate timeseries types from '/home/eliza/Code/oxide/omicron/oximeter/impl/../oximeter/schema/sensor-measurement.toml': Toml("TOML parse error at line 8, column 6\n  |\n8 |     {\n  |      ^\ninvalid inline table\nexpected `}`\n")
 --> gateway/src/metrics.rs:7:27
  |
7 | oximeter::use_timeseries!("sensor-measurement.toml");
  |                           ^^^^^^^^^^^^^^^^^^^^^^^^^

Note that the toml crate very kindly tries to give us a nice ASCII graphic pointing at the location where the parse error occurred, but because the strings are formatted using fmt::Debug, it's not particularly comprehensible.

This commit changes the macro to use fmt::Display, instead, so that the multi-line diagnostic output from the TOML parser is more readable. Now, it looks like this:

error: Failed to generate timeseries types from '/home/eliza/Code/oxide/omicron/oximeter/impl/../oximeter/schema/sensor-measurement.toml':
       TOML deserialization error: TOML parse error at line 8, column 6
         |
       8 |     {
         |      ^
       invalid inline table
       expected `}`
 --> gateway/src/metrics.rs:7:27
  |
7 | oximeter::use_timeseries!("sensor-measurement.toml");
  |                           ^^^^^^^^^^^^^^^^^^^^^^^^^

I cherry-picked this change from #6354, since it seemed like a self-contained change that could easily land on its own.

[bnaecker]

Much nicer, thanks!