`MetricSample` had no `__str__`, so it fell back to the verbose
dataclass `__repr__`. Add a compact one rendering `{metric}={value}`,
with `@{connection}` appended when a connection is set. The metric
follows the same convention as `MetricConnection.__str__`: a known
member renders as its name, the unspecified sentinel (`0` /
`UNSPECIFIED`) as `<invalid:0>`, and an unknown int bare.

`MetricSample` is the `instance` reported by `get_metric()`
(`Unrecognized` / `UnspecifiedEnumValueError`) and `get_bounds_set()`
(`InvalidBoundsSetError`); without a `__str__` those messages embedded
the whole dataclass repr. A short representation keeps the errors
readable.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

The default (and custom) messages of the accessor errors interpolated
the offending value with `!r`. Switch the non-string values to plain
`{value}` so wrapper values render through their compact `__str__` —
e.g. `<invalid:[10.0,-10.0]>` instead of `InvalidBounds(lower=10.0,
upper=-10.0)` — which surfaces the `<invalid:...>` markers in the
message and keeps it readable.

Covered: `UnrecognizedEnumValueError`, `InvalidLatitudeError`,
`InvalidLongitudeError`, `InvalidLifetimeError`, `InvalidBoundsError`,
`InvalidBoundsSetError` and `InvalidDeliveryAreaError`, plus the custom
messages raised by `ElectricalComponent.provides_telemetry()`,
`accepts_control()` and `get_metric_config_bounds()`, and
`Microgrid.is_active()`.

String values keep `!r`: an invalid `country_code` may carry surprising
characters, and quoting keeps them visible. The `attr_name` likewise
keeps `!r`, the idiomatic way to show an attribute name.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>